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Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg
Abstract
We present the results of a large multi-wavelength follow-up campaign of the Tidal Disruption Event (TDE) AT 2019dsg, focusing on low to high resolution optical spectroscopy, X-ray, and radio observations. The galaxy hosts a super massive black hole of mass $\rm (5.4\pm 3.2)\times 10^6\, M_\odot$ and careful analysis finds no evidence for the presence of an Active Galactic Nucleus, instead the TDE host galaxy shows narrow optical emission lines that likely arise from star formation activity. The transient is luminous in the X-rays, radio, UV and optical. The X-ray emission becomes undetected after ∼100 days, and the radio luminosity density starts to decay at frequencies above 5.4 GHz by ∼160 days. Optical emission line signatures of the TDE are present up to ∼200 days after the light curve peak. The medium to high resolution spectra show traces of absorption lines that we propose originate in the self-gravitating debris streams. At late times, after ∼200 days, narrow Fe lines appear in the spectra. The TDE was previously classified as N-strong, but after careful subtraction of the host galaxy’s stellar contribution, we find no evidence for these N lines in the TDE spectrum, even though O Bowen lines are detected. The observed properties of the X-ray emission are fully consistent with the detection of the inner regions of a cooling accretion disc. The optical and radio properties are consistent with this central engine seen at a low inclination (i.e., seen from the poles).
... Therefore, a similar amount of energy is expected to be injected into neutrinos and the electromagnetic (EM) cascade in the source, where it is a priori not clear where in the energy the EM signatures dominate, and what the timedependent behavior of the EM cascade is-these questions require dedicated theoretical modeling, which we perform in this study. A joint analysis of the nonthermal X-ray and γ-ray observations with the neutrino detection will then provide further insights: the X-ray light curve for AT2019dsg was measured using Swift X-ray Telescope (XRT; Burrows et al. 2005), XMM-Newton (XMM; Jansen et al. 2001), and NICER (Cannizzaro et al. 2021;Stein et al. 2021). As for AT2019fdr, upper limits were obtained from XRT and eROSITA (Saxton et al. 2020;Predehl et al. 2021;Reusch et al. 2022). ...
... The the magenta, green, orange, and blue solid curves illustrate the single-flavor neutrino (L d 4 L 2 p n ), very high-energy (VHE) γ-ray (0.3-100 TeV), γ-ray (0.1-800 GeV), and X-ray light curves, respectively. The flux upper limits from HAWC, Fermi, and XRT/XMM/NICER or eROSITA (Cannizzaro et al. 2021;Predehl et al. 2021;Stein et al. 2021;Reusch et al. 2022) are shown as the green, orange, and blue points, respectively. Furthermore, the neutrino detection times are marked as vertical magenta dotted curves. ...
... The rapid (exponential) decay of observed X-ray light curve in the time interval 0 t − t pk 50 days (illustrated as the blue dashed line in the upper right panel of Figure 3) might be caused by the cooling of the accretion disk or by dust obscuration. Interestingly, there is an additional X-ray (Swift-XRT) data point around t − t pk = 100 days that was identified in Cannizzaro et al. (2021) and does not fit this rapid decay picture. Notably, the X-ray cascade emission can describe that data point, which means that Swift may have actually seen an electromagnetic cascade signature there. ...
The potential association between Tidal Disruption Events and high-energy astrophysical neutrinos implies the acceleration of cosmic rays. These accelerated particles will initiate electromagnetic (EM) cascades spanning from keV to GeV energies via the processes related to neutrino production. We model the EM cascade and neutrino emissions by numerically solving the time-dependent transport equations, and discuss the implications for AT2019dsg and AT2019fdr in the X-ray and γ -ray bands. We show that the γ -ray constraints from Fermi can constrain the size of the radiation zone and the maximum energy of injected protons, and that the corresponding expected neutrino event numbers in follow-up searches are limited to be less than about 0.1. Depending on the efficiency of p γ interactions and the time at which the target photons peak, the X-ray and γ -ray signals can be expected closer to the peak of the optical-ultraviolet luminosity or to the time of the neutrino production.
... Therefore, a similar amount of energy is expected to be injected into neutrinos and the electromagnetic (EM) cascade in source, where it is a priori not clear where in energy the EM signatures dominate, and what the timedependent behavior of the EM cascade is -these questions require dedicated theoretical modeling, which we perform in this study. A joint analysis of the nonthermal X-ray and γ-ray observations with the neutrino detection will then provide further insights: The X-ray light curve for AT2019dsg was measured using Swift X-ray Telescope (XRT, Burrows et al. 2005), XMM-Newton (XMM, Jansen et al. 2001), and NICER (Stein et al. 2021;Cannizzaro et al. 2021). As for AT2019fdr, upper limits were obtained from XRT and eROSITA (Saxton et al. 2020;Reusch et al. 2022;Predehl et al. 2021). ...
... The the magenta, green, orange, and blue solid curves illustrate the single-flavor neutrino (Lν /4πd 2 L ), very-high-energy (VHE) γ-ray (0.3 -100 TeV), γ-ray (0.1 -800 GeV), and X-ray light curves, respectively. The flux up limits from HAWC, Fermi, and XRT/XMM/NICER or eROSITA (Stein et al. 2021;Cannizzaro et al. 2021;Predehl et al. 2021;Reusch et al. 2022) are shown as the green, orange, and blue points, respectively. Furthermore, the neutrino detection times are marked as the vertical magenta dotted curves. ...
... The rapid (exponential) decay of observed X-ray light curve in the time interval 0 ∼ < t − t pk ∼ < 50 d (illustrated as the blue dashed line in the upper right panel of Fig. 3) might be caused by the cooling of the accretion disk or by dust obscuration. Interestingly, there is an additional X-ray (Swift-XRT) data point around t − t pk = 100 d which was identified in Cannizzaro et al. (2021) and does not fit this rapid decay picture. Notably, the X-ray cascade emission can describe that data point, which means that Swift may have actually seen an electromagnetic cascade signature there. ...
The potential association between Tidal Disruption Events (TDEs) and high-energy astrophysical neutrinos implies the acceleration of cosmic rays. These accelerated particles will initiate electromagnetic (EM) cascades spanning from keV to GeV energies by the processes related to neutrino production. We model the EM cascade and neutrino emissions by numerically solving the time-dependent transport equations and discuss the implications for AT2019dsg and AT2019fdr in the X-ray and $\gamma$-ray bands. We show that the $\gamma$-ray constraints from \emph{Fermi} can constrain the size of the radiation zone and the maximum energy of injected protons, and that the corresponding expected neutrino event numbers in follow-up searches are limited to be less than about 0.1. Depending on the efficiency of $p\gamma$ interactions, the X-ray and $\gamma$-ray signals can be expected closer to the peak of the optical-ultraviolet (OUV) luminosity, or to the time of the neutrino production.
... The optical spectra are characterized by a strong blue continuum at early times and are dominated by broad ( ∼10 4 km s −1 ) H and/or He emission lines with different strengths and relative ratios (Arcavi et al. 2014 ;Leloudas et al. 2019 ;Charalampopoulos et al. 2022 ). A fraction of TDEs have shown broad Bowen fluorescence emission lines (Blagorodnova et al. 2019 ;Leloudas et al. 2019 ;Onori et al. 2019 ) and evidence for Fe II emission lines have been found in a small subset of TDEs (Wevers et al. 2019b ;Cannizzaro et al. 2021 ). Following these discoveries, the TDE population has been divided into three main spectral classes, depending on the appearance or lack of the different broad spectral features (Arcavi et al. 2014 ;Leloudas et al. 2019 ;van Velzen et al. 2020 ). ...
... Despite what is expected in the case of emission from a newly formed accretion disc, TDEs selected in the optical are typically not detected in the X-rays. Only few exceptions have been discovered, with some events showing also soft X-ray emission, sometimes delayed with respect to the optical peak emission (e.g ASASSN-14li, ASASSN-15oi, A T 2019dsg, A T 2018fyk, A T 2019qiz, A T 2019azh; Holoien et al. 2016 ;Gezari, Cenko & Arcavi 2017 ;Wevers et al. 2019b ;Nicholl et al. 2020 ;Cannizzaro et al. 2021 ;Liu et al. 2022 , respectively) Despite the recent progress in this field, there are many aspects that remain unclear, such as the emission mechanism behind all the observed features, the geometry of the emitting region and the X-ray non-detection of the optically selected TDEs. Different scenarios have been proposed, including the presence of an optically thick atmosphere which reprocess the high-energy radiation emitted during the accretion process (Guillochon, Manukian & Ramirez-Ruiz 2014 ;Roth et al. 2016 ;Roth & Kasen 2018 ), or emission by shocks from intersecting stellar debris streams during the disc formation phase Shiokawa et al. 2015 ;Bonnerot, Rossi & Lodato 2017 ). ...
... The non-detection of the He II in the optical spectra taken prior to the X-ray flare may indicate the absence of the X-ray emission at these epochs, as the He II line is known to be correlated with the soft X-ray photons (Pakull & Angebault 1986 ;Schaerer, Fragos & Izotov 2019 ;Cannizzaro et al. 2021 ). Ho we ver, we note that in the case of the TDE ASASSN-15oi and AT 2019azh a broad He II feature is detected well before the onset of a delayed soft X-ray flare Hinkle et al. 2021 ;van Velzen et al. 2021b ). ...
We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT 2017gge, covering a total of 1698 days from the transient’s discovery. The bolometric light-curve, the blackbody temperature and radius, the broad H and He i λ5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of ∼200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He ii λ4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He i λ10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the [Fe xiii] λ10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.
... The optical spectra are characterized by a strong blue continuum at early times and are dominated by broad (∼10 4 km s −1 ) H and/or He emission lines with different strengths and relative ratios (Arcavi et al. 2014;Leloudas et al. 2019;Charalampopoulos et al. 2022). A fraction of TDEs have shown broad Bowen fluorescence emission lines (Blagorodnova et al. 2019;Leloudas et al. 2019;Onori et al. 2019) and evidence for Fe emission lines have been found in a small subset of TDEs (Wevers et al. 2019b;Cannizzaro et al. 2021). Following these discoveries, the TDE population has been divided into three main spectral classes, depending on the appearance or lack of the different broad spectral features (Arcavi et al. 2014;Leloudas et al. 2019;van Velzen et al. 2020). ...
... Despite what is expected in the case of emission from a newly formed accretion disk, TDEs selected in the optical are not detected in the X-rays. Only few exceptions have been discovered, with some events showing also soft X-ray emission, sometimes delayed with respect to the optical peak emission (e.g ASASSN-14li, ASASSN-15oi, AT 2019dsg, AT 2018fyk, AT 2019qiz, AT 2019azh;Holoien et al. 2016;Gezari et al. 2017;Cannizzaro et al. 2021;Wevers et al. 2019b;Nicholl et al. 2020;Liu et al. 2022, respectively). ...
... The non-detection of the He in the optical spectra taken prior to the X-ray flare strongly suggests the absence of the X-ray emission at these epochs, as the He line is known to be correlated with the soft X-ray photons (Pakull & Angebault 1986;Schaerer et al. 2019;Cannizzaro et al. 2021). Additionally, in the recent work of Nicholl et al. (2022), where the light-curves of 32 optically bright TDEs have been thoroughly analyzed, it has been found that the events without He were consistent with a stream-crossing origin for the luminosity, while those with He were more consistent with forming accretion disks. ...
We present the results from a dense multi-wavelength (optical/UV, IR, and X-ray) follow-up campaign of the nuclear transient AT 2017gge, covering a total of 1698 days from the transient's discovery. The bolometric light-curve, the black body temperature and radius, as well as the broad H and He I $\lambda$5876 emission lines and their evolution with time, are all consistent with a TDE nature. A soft X-ray flare is detected with a delay of ~200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II $\lambda$4686 and by a number of long-lasting high ionization coronal emission lines. An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I $\lambda$10830. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.
... Of those TDEs that are detected in radio (e.g., Alexander et al. 2020;Horesh et al. 2021), only a few have been imaged using VLBI, with large differences in radio luminosity evolution, environmental properties, and the level of collimation of the outflow. Besides, extensive monitoring campaigns have only been conducted on Swift J1644+5734 (e.g., Levan et al. 2011;Zauderer et al. 2011;Berger et al. 2012;Zauderer et al. 2013;Yang et al. 2016;Eftekhari et al. 2018;Cendes et al. 2021b) and Arp 299-B AT1 (e.g., Mattila et al. 2018) owing to the presence of a relativistic jet, ASASSN-15oi (Horesh et al. 2021) due to a peculiarly delayed radio flaring (over timescales of months to years), ASASSN-14li (e.g., Romero-Cañizales et al. 2016;van Velzen et al. 2016;Bright et al. 2018;Alexander et al. 2016) being relatively close in proximity (z = 0.02), and AT2019dsg owing to its persistent radio emission and potential neutrino association (Stein et al. 2021;Cannizzaro et al. 2021;Cendes et al. 2021a;Matsumoto et al. 2022). A multiwavelength observation campaign was carried out to study AT2019dsg and its host galaxy (Cannizzaro et al. 2021). ...
... Besides, extensive monitoring campaigns have only been conducted on Swift J1644+5734 (e.g., Levan et al. 2011;Zauderer et al. 2011;Berger et al. 2012;Zauderer et al. 2013;Yang et al. 2016;Eftekhari et al. 2018;Cendes et al. 2021b) and Arp 299-B AT1 (e.g., Mattila et al. 2018) owing to the presence of a relativistic jet, ASASSN-15oi (Horesh et al. 2021) due to a peculiarly delayed radio flaring (over timescales of months to years), ASASSN-14li (e.g., Romero-Cañizales et al. 2016;van Velzen et al. 2016;Bright et al. 2018;Alexander et al. 2016) being relatively close in proximity (z = 0.02), and AT2019dsg owing to its persistent radio emission and potential neutrino association (Stein et al. 2021;Cannizzaro et al. 2021;Cendes et al. 2021a;Matsumoto et al. 2022). A multiwavelength observation campaign was carried out to study AT2019dsg and its host galaxy (Cannizzaro et al. 2021). The results include the inference of a central SMBH of 5.4 × 10 6 M e , derived from the optical spectroscopy. ...
... The compiled 5 GHz light curve consists of flux densities reported in Stein et al. (2021) and Cannizzaro et al. (2021), as well as the measurements from our EVN observations. A smooth broken power law is used to fit the light curve (see Appendix B and Table 4 for the fit-parameter estimates), presented in Figure 2. The peak flux density n F p is inferred to be 1.18 ± 0.18 mJy at a time t p of 152.8 ± 16.2 days. ...
A tidal disruption event (TDE) involves the shredding of a star in the proximity of a supermassive black hole (SMBH). The nearby (≈230 Mpc) relatively radio-quiet, thermal-emission-dominated source AT2019dsg is the first TDE with a potential neutrino association. The origin of nonthermal emission remains inconclusive; possibilities include a relativistic jet or a subrelativistic outflow. Distinguishing between them can address neutrino production mechanisms. High-resolution very long baseline interferometry 5 GHz observations provide a proper motion of 0.94 ± 0.65 mas yr ⁻¹ (3.2 ± 2.2 c ; 1 σ ). Modeling the radio emission favors an origin from the interaction between a decelerating outflow (velocity ≈0.1 c ) and a dense circumnuclear medium. The transition of the synchrotron self-absorption frequency through the observation band marks a peak flux density of 1.19 ± 0.18 mJy at 152.8 ± 16.2 days. An equipartition analysis indicates an emission-region distance of ≥ 4.7 × 10 ¹⁶ cm, magnetic field strength ≥ 0.17 G , and number density ≥ 5.7 × 10 ³ cm ⁻³ . The disruption involves a ≈2 M ⊙ star with a penetration factor ≈1 and a total energy output of ≤ 1.5 × 10 ⁵² erg. The outflow is radiatively driven by the accretion of stellar debris onto the SMBH. Neutrino production is likely related to the acceleration of protons to peta-electron-volt energies and the availability of a suitable cross section at the outflow base. The present study thus helps exclude jet-related origins for nonthermal emission and neutrino production, and constrains nonjetted scenarios.
... Mattila et al. 2018) owing to the presence of a relativistic jet, ASASSN-15oi (Horesh et al. 2021) due to a peculiarly delayed radio flaring (over months to years timescales), ASASSN-14li (e.g. van Velzen et al. 2016;Alexander et al. 2016;Romero-Cañizales et al. 2016;Bright et al. 2018) being relatively close proximity (z = 0.02), and AT2019dsg owing to its persistent radio emission and potential neutrino association (Stein et al. 2021;Cannizzaro et al. 2021;Cendes et al. 2021a;Matsumoto et al. 2021). A multi-wavelength observation campaign was carried out to study AT2019dsg and its host galaxy (Cannizzaro et al. 2021). ...
... van Velzen et al. 2016;Alexander et al. 2016;Romero-Cañizales et al. 2016;Bright et al. 2018) being relatively close proximity (z = 0.02), and AT2019dsg owing to its persistent radio emission and potential neutrino association (Stein et al. 2021;Cannizzaro et al. 2021;Cendes et al. 2021a;Matsumoto et al. 2021). A multi-wavelength observation campaign was carried out to study AT2019dsg and its host galaxy (Cannizzaro et al. 2021). The results include the inference of a central SMBH of 5.4 × 10 6 M , derived from the optical spectroscopy. ...
... The compiled 5-GHz light curve consists of flux densities reported in Stein et al. (2021) and Cannizzaro et al. (2021), as well as the measurements from our EVN observations. A smooth broken power law is used to fit the light curve (see Appendix B and Table 4 for the fit parameter estimates), presented in Figure 2. The peak flux density F νp is inferred to be 1.18 ± 0.18 mJy at a time t p of 152.8 ± 16.2 days. ...
A tidal disruption event (TDE) involves the shredding of a star in the proximity of a supermassive black hole (SMBH). The nearby (230 Mpc) relatively radio-quiet, thermal emission dominated source AT2019dsg is the first TDE with a potential neutrino association. The origin of non-thermal emission remains inconclusive; possibilities include a relativistic jet or a sub-relativistic outflow. Distinguishing between them can address neutrino production mechanisms. High-resolution very long baseline interferometry 5-GHz observations provide a proper motion of 0.94 ± 0.65 mas/yr (3.2 ± 2.2 c; 1 − σ). Modelling the radio emission favors an origin from the interaction between a decelerating outflow (velocity ~ 0.1 c) and a dense circum-nuclear medium. The transition of the synchrotron self-absorption frequency through the observation band marks a peak flux density of 1.19 ± 0.18 mJy at 152.8 ± 16.2 days. An equipartition analysis indicates an emission region distance of 4.7 x 10^(16) cm, magnetic field strength 0.17 G, and number density 5.7 x 10^(3) /cm^(3). The disruption involves a ~2 solar mass star with a penetration factor ~ 1 and a total energy output of 1.5 x 10^(52) erg. The outflow is radiatively driven by accretion of stellar debris onto the SMBH. Neutrino production is likely related to the acceleration of protons to PeV energies and the availability of a suitable cross-section at the outflow base. The present study thus helps exclude jet-related origins for non-thermal emission and neutrino production, and constrains non-jetted scenarios.
... Its peak optical luminosity (∼10 44.5 erg s −1 ) places it in the top 10% of optical TDEs to date (van Velzen et al. 2021), and a high level of optical polarization was observed at early times (Lee et al. 2020). AT2019dsg also exhibited X-ray emission and radio emission in the first few months following discovery (Cannizzaro et al. 2021;Stein et al. 2021). Additionally, Stein et al. (2021) claim a potential coincident high-energy neutrino with the spatial location of AT2019dsg, but several months after discovery (on 2019 October 1); the emission mechanism for such a neutrino is debated (Fang et al. 2020;Liu et al. 2020;Murase et al. 2020;Winter & Lunardini 2021). ...
... We fit the SEDs 8 with the model developed by Granot & Sari (2002) for synchrotron emission from gamma-ray burst (GRB) afterglows, specifically in the regime where ν m = ν a ; this is relevant for non-relativistic sources as validated by the analysis below. We have chosen the stellar wind (k = 2) solutions from Granot & Sari (2002) as the closest 7 We compared our VLA data to the e-MERLIN observations at 5.1 GHz, some of which overlap with our observations (Cannizzaro et al. 2021). The September 7 e-MERLIN flux densities appear systematically higher than our own. ...
... SMBH's Schwarzschild radius. For AT2019dsg we use the value of = M log 6.7 0.4 SMBH M e , or R s = (1.6 ± 0.9) × 10 12 cm (Cannizzaro et al. 2021). With this value, our observations span a scale of ≈(5-44) × 10 3 R s . ...
We present detailed radio observations of the tidal disruption event (TDE) AT2019dsg, obtained with the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), and spanning 55-560 days post disruption. We find that the peak brightness of the radio emission increases until ~200 days and subsequently begins to decrease steadily. Using a standard equipartition analysis, including the effects of synchrotron cooling as determined by the joint VLA-ALMA spectral energy distributions, we find that the outflow powering the radio emission is in roughly free expansion with a velocity of ˜0.07 c, while its kinetic energy increases by a factor of about 5 from 55 to 200 days and plateaus at ˜4.4 1048 erg thereafter. The ambient density traced by the outflow declines as radius ˜R -1.7 on a scale of ˜(1-4) 1016 cm (˜6300-25,000 R s ), followed by a steeper decline to ˜7 1016 cm (˜44,000 R s ). Allowing for a collimated geometry, we find that to reach even mildly relativistic velocities (G = 2) the outflow requires an opening angle of ? j ˜ 2 , which is narrow even by the standards of gamma-ray burst jets; a truly relativistic outflow requires an unphysically narrow jet. The outflow velocity and kinetic energy in AT2019dsg are typical of previous non-relativistic TDEs, and comparable to those from type Ib/c supernovae, raising doubts about the claimed association with a high-energy neutrino event. © 2021. The Author(s). Published by the American Astronomical Society..
... As follows from Eq. (2), the neutrino production depends strongly on SMBH . For AT2019dsg estimates of this quantity are methoddependent, varying between 1.3 10 6 [10] and 3 10 7 [2] ; see also [11] for intermediate values. Considering this uncertainty, our arguments above leave two possible conclusions: ...
... A key observation of AT2019dsg is a quickly fading X-ray luminosity (solid blue curve in Fig. 1); exponentially decaying over the timescale of tens of days. The origin of the decay may be the outflow obscuring the X-rays -consistent with Ref. [12] in terms of the optical thickness at the radii of interest -or the cooling of the accretion disk, which shifts the peak of the X-ray spectrum out of the Swift energy window, see Ref. [11], or both. We anticipate that a small fraction (∼10%) of the X-rays isotropize in the region relevant for the neutrino production, see Fig. 1, where the time evolution of the unattenuated X-rays follows the model in [13]. ...
... In addition, we expect efficient particle acceleration in the jet, and our requirements for the acceleration efficiency are moderate. Although a relativistic jet has not been directly observed, there are some indications from optical polarimetry measurements [24], and even the radio signal may be interpreted as due to a relativistic jet in non-vanilla models [11]. In Ref. [11], a late-term X-ray signal at about 115 days after discovery has been found as well, which does not fit the exponential decay trend described above; one may speculate that this could be a signature of the jetted radiation. ...
The observation of a neutrino at IceCube in association with the Tidal Disruption Event (TDE) AT2019dsg has suggested TDEs as a new class of sources of astrophysical neutrinos. We present a model of this multi-messenger observation in a jetted concordance scenario, where the neutrino production is directly linked to the observed X-rays, and the timing of the neutrino observation (about 150 days post peak) can be naturally described. We briefly discuss the implications of our model for future neutrino-TDE associations.
... Besides, extensive monitoring campaigns have only been conducted on Swift J1644+5734 9, 21-26 and Arp 299-B AT1 7 owing to the presence of a relativistic jet, and ASASSN-14li 27-30 being relatively close proximity (z = 0.02). A multi-wavelength observation campaign was carried out to study AT2019dsg and its host galaxy 31 . The results include the inference of a central SMBH of 5.4 × 10 6 M , derived from the optical spectroscopy. ...
... Light curve fitting The compiled 5-GHz light curve (see Figure 2) includes data points covering the rising phase and peak 14,31 , our three VLBI points and a final point in the declining phase 19 . ...
... The latter is the only data point used from Ref [ 19 ]. This is as they note that their VLA flux densities are systematically lower than the e-MERLIN flux densities 31 . Since our VLBI estimates are likely to be closer to the e-MERLIN values, we mainly use the reported flux densities reported in Ref [ 14 ] and Ref [ 31 ]. ...
A tidal disruption event (TDE) involves the tidal shredding of a star in the vicinity of a dormant supermassive black hole. The nearby (~230 mega-parsec) radio-quiet (radio luminosity of 4x10^(38) erg/s) AT2019dsg is the first TDE potentially associated with a neutrino event. The origin of the non-thermal emission in AT2019dsg remains inconclusive; possibilities include a relativistic jet or a sub-relativistic outflow. Distinguishing between them can address neutrino production mechanisms. High resolution very long baseline interferometry monitoring provides uniquely constraining flux densities and proper motion of the ejecta. A non-relativistic (outflow velocity of ~0.1 c) decelerated expansion in a relatively dense environment is found to produce the radio emission. Neutrino production may be related to the acceleration of protons by the outflow. The present study thus helps exclude jet-related origins for the non-thermal emission and neutrino production, and constrains non-jetted scenarios .
... As follows from Eq. (2), the neutrino production depends strongly on SMBH . For AT2019dsg estimates of this quantity are methoddependent, varying between 1.3 10 6 [10] and 3 10 7 [2] ; see also [11] for intermediate values. Considering this uncertainty, our arguments above leave two possible conclusions: ...
... A key observation of AT2019dsg is a quickly fading X-ray luminosity (solid blue curve in Fig. 1); exponentially decaying over the timescale of tens of days. The origin of the decay may be the outflow obscuring the X-rays -consistent with Ref. [12] in terms of the optical thickness at the radii of interest -or the cooling of the accretion disk, which shifts the peak of the X-ray spectrum out of the Swift energy window, see Ref. [11], or both. We anticipate that a small fraction (∼10%) of the X-rays isotropize in the region relevant for the neutrino production, see Fig. 1, where the time evolution of the unattenuated X-rays follows the model in [13]. ...
... In addition, we expect efficient particle acceleration in the jet, and our requirements for the acceleration efficiency are moderate. Although a relativistic jet has not been directly observed, there are some indications from optical polarimetry measurements [24], and even the radio signal may be interpreted as due to a relativistic jet in non-vanilla models [11]. In Ref. [11], a late-term X-ray signal at about 115 days after discovery has been found as well, which does not fit the exponential decay trend described above; one may speculate that this could be a signature of the jetted radiation. ...
... Remarkably, the X-rays brightened by one order of magnitude 200 days later, followed by a flare in the radio [9][10][11][12] . Finally, we obtained data for AT 2019dsg, a TDE possibly associated with a high-energy neutrino 13 and showing narrow emission lines in its spectrum 14 at z = 0.052. X-rays at a level of L X ≈ 2.5 × 10 43 erg s −1 were detected from AT 2019dsg at early phases but faded rapidly, while the source also showed persistent radio emission. ...
... Focusing on Hα, we observe that the core of the emission line is indeed partially depolarized, but the wings of the lines (velocities ≈ 20,000 km s −1 ) show polarization peaks, especially prominent for AT 2018dyb and AT 2019azh. The profile of AT 2019dsg is more complex, but this event is also different spectroscopically, showing both a broad and a narrow component that does not evolve with time 14 . It is possible that the host of AT 2019dsg contains an AGN, which can contribute to the polarization by a hidden broadlined region, similar to what has been observed in Seyfert 2 galaxies 5 . ...
A star crossing the tidal radius of a supermassive black hole will be spectacularly ripped apart with an accompanying burst of radiation. A few tens of such tidal disruption events have now been identified in optical wavelengths, but the exact origin of the strong optical emission remains inconclusive. Here we report polarimetric observations of three tidal disruption events. The continuum polarization appears independent of wavelength, while emission lines are partially depolarized. These signatures are consistent with photons being scattered and polarized in an envelope of free electrons. An almost axisymmetric photosphere viewed from different angles is in broad agreement with the data, but there is also evidence for deviations from axial symmetry before the peak of the flare and significant time evolution at early times, compatible with the rapid formation of an accretion disk. By combining a super-Eddington accretion model with a radiative transfer code, we simulate the polarization degree as a function of disk mass and viewing angle and we show that the predicted levels are compatible with the observations for extended reprocessing envelopes of ~1,000 gravitational radii. Spectropolarimetry therefore constitutes a new observational test for tidal disruption event models, and opens an important new line of exploration in the study of tidal disruption events.
... Remarkably, the X-rays brightened by one order of magnitude 200 days later, followed by a flare in the radio 9,10,11,12 . Finally, we obtained data for AT 2019dsg, a TDE possibly associated with a high-energy neutrino 13 , and showing narrow emission lines in its spectrum 14 at z = 0.052. X-rays at a level of L X ∼ 2.5 × 10 43 erg s −1 were detected from AT 2019dsg at early phases but faded rapidly, while the source also showed persistent radio emission. ...
... Focusing on Hα, we observe that the core of the emission line is indeed partially depolarized, but the wings of the lines (velocities ∼ 20,000 km s −1 ) show polarization peaks, especially prominent for AT 2018dyb and AT 2019azh. The profile of AT 2019dsg is more complex, but this event is also different spectroscopically, showing both a broad and a narrow component that does not evolve with time 14 . It is possible that the host of AT 2019dsg contains an AGN, which can contribute in the polarization by a hidden broadlined region, similar to what has been observed in Seyfert 2 galaxies 5 . ...
A star crossing the tidal radius of a supermassive black hole will be spectacularly ripped apart with an accompanying burst of radiation. A few tens of such tidal disruption events (TDEs) have now been identified in the optical wavelengths, but the exact origin of the strong optical emission remains inconclusive. Here we report polarimetric observations of three TDEs. The continuum polarization is independent of wavelength, while emission lines are partially depolarized. These signatures are consistent with optical photons being scattered and polarized in an envelope of free electrons. An almost axisymmetric photosphere viewed from different angles is in broad agreement with the data, but there is also evidence for deviations from axial symmetry before the peak of the flare and significant time evolution at early times, compatible with the rapid formation of an accretion disk. By combining a super-Eddington accretion model with a radiative transfer code we generate predictions for the degree of polarization as a function of disk mass and viewing angle, and we show that the predicted levels are compatible with the observations, for extended reprocessing envelopes of $\sim$1000 gravitational radii. Spectropolarimetry therefore constitutes a new observational test for TDE models, and opens an important new line of exploration in the study of TDEs.
... From the top panel, we can see that the SED of ASASSN-14li is consistent with an accretion disk even at early times; very little to no UV/optical excess is present; furthermore, the evolution in SED shape with time is very small and compatible with the cooling of the accretion disk. The same holds for the other Power-law decaying source, AT2019dsg, although the rapid decay of X-ray emission indicates a quicker cooling of the accretion disk (Cannizzaro et al. 2021). ...
We present a systematic analysis of the X-ray emission of a sample of 17 optically selected, X-ray-detected tidal disruption events (TDEs) discovered between 2014 and 2021. The X-ray light curves show a diverse range of temporal behaviors, with most sources not following the expected power-law decline. The X-ray spectra are mostly extremely soft and consistent with thermal emission from the inner region of an accretion disk that cools as the accretion rate decreases. Three sources show the formation of a hard X-ray corona around 200 days after the UV/optical peak. The shape of the spectral energy distribution, traced by the ratio ($L_{\rm BB}/L_{\rm X}$) between the UV/optical and X-ray luminosities, shows a wide range $L_{\rm BB}/L_{\rm X} \in (0.5, 3000)$ at early-times, and converges to disk-like values $L_{\rm BB}/L_{\rm X} \in (0.5, 10)$ at late-times. The evolution of the derived physical parameters favors a decrease in the optical depth of a reprocessing layer instead of delayed disk formation to explain the late-time X-ray brightening found in several sources. We estimate the fraction of optically discovered TDEs with $L_{\rm X}\geq 10^{42}$ erg s$^{-1}$ to be at least $40\%$, and find that the X-ray loudness is independent of black hole mass. We combine our sample with those from X-ray surveys to construct an X-ray luminosity function, best fitted by a broken power-law with a brake at $\sim 10^{44}$ erg s$^{-1}$. We show that there is no dichotomy between optically and X-ray selected TDEs; instead, there is a continuum of early time $L_{\rm BB}/L_{\rm X}$, at least as wide as $L_{\rm BB}/L_{\rm X} \in (0.1, 3000)$, with optical/X-ray surveys selecting preferentially, but not exclusively, from the higher/lower end of the distribution. Our findings are consistent with an orientation-dependent and time-evolving reprocessing layer, and support viewing-angle unification models.
... AT 2021lwx has narrower H Balmer line profiles compared to broader ones seen in typical TDEs (Holoien et al. 2014;Brown et al. 2017;Hung et al. 2021;Van Velzen et al. 2021). However, there are exceptions to this scenario as is noted in the case of PS16dtm (Blanchard et al. 2017) and other TDEs (AT 2019dsg; Cannizzaro et al. 2021Van Velzen et al. 2021). No BF emission lines are seen in the spectra of AT 2021lwx, unlike some TDEs. ...
We report multiwavelength observations and characterization of the ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka “Barbie”) identified in the alert stream of the Zwicky Transient Facility (ZTF) using a Recommender Engine For Intelligent Transient Tracking filter on the ANTARES alert broker. From a spectroscopically measured redshift of 0.995, we estimate a peak-observed pseudo-bolometric luminosity of log( L max / [ erg s − 1 ] ) = 45.7 from slowly fading ztf- g and ztf- r light curves spanning over 1000 observer-frame days. The host galaxy is not detected in archival Pan-STARRS observations ( g > 23.3 mag), implying a lower limit to the outburst amplitude of more than 5 mag relative to the quiescent host galaxy. Optical spectra exhibit strong emission lines with narrow cores from the H Balmer series and ultraviolet semi-forbidden lines of Si iii ] λ 1892, C iii ] λ 1909, and C ii ] λ 2325. Typical nebular lines in Active Galactic Nucleus (AGN) spectra from ions such as [O ii ] and [O iii ] are not detected. These spectral features, along with the smooth light curve that is unlike most AGN flaring activity and the luminosity that exceeds any observed or theorized supernova, lead us to conclude that AT 2021lwx is most likely an extreme tidal disruption event (TDE). Modeling of ZTF photometry with MOSFiT suggests that the TDE was between a ≈14 M ⊙ star and a supermassive black hole of mass M BH ∼ 10 ⁸ M ⊙ . Continued monitoring of the still-evolving light curve along with deep imaging of the field once AT 2021lwx has faded can test this hypothesis and potentially detect the host galaxy.
... For AT2019dsg, jets (Liu et al. 2020;Winter & Lunardini 2021), outflow-cloud interactions (Wu et al. 2022), disk, corona, hidden winds, or jets (Murase et al. 2020b) have been proposed (see Hayasaki 2021 for an overview). While a collimated outflow, such as a jet, has the advantage that it can provide the necessary power for the neutrino emission (see discussion in Winter & Lunardini 2022), no convincing direct jet signatures for AT2019dsg have been observed (Mohan et al. 2022), and the observed radio signal might only be interpreted as a jet signature in scenarios with purely leptonic radiative signatures for an unnaturally narrow jet (Cendes et al. 2021) or a steep density profile (Cannizzaro et al. 2021). For AT2019fdr, corona, hidden wind, and jet models have been considered in Reusch et al. (2022), and in van Velzen et al. (2021a), a disk model for all three TDEs has been proposed. ...
Three Tidal Disruption Event candidates (AT2019 dsg , AT2019 fdr , and AT2019 aalc ) have been associated with high-energy astrophysical neutrinos in multimessenger follow-ups. In all cases, the neutrino observation occurred ( 100 ) days after the maximum of the optical-ultraviolet (OUV) luminosity. We discuss unified fully time-dependent interpretations of the neutrino signals where the neutrino delays are not a statistical effect, but rather the consequence of a physical scale of the post-disruption system. Noting that X-ray flares and infrared (IR) dust echoes have been observed in all cases, we consider three models in which quasi-isotropic neutrino emission is due to the interactions of accelerated protons of moderate, medium, and ultra-high energies with X-rays, OUV, and IR photons, respectively. We find that the neutrino time delays can be well described in the X-ray model assuming magnetic confinement of protons in a calorimetric approach if the unobscured X-ray luminosity is roughly constant over time, and in the IR model, where the delay is directly correlated with the time evolution of the echo luminosity (for which a model is developed here). The OUV model exhibits the highest neutrino production efficiency. In all three models, the highest neutrino fluence is predicted for AT2019 aalc , due to its high estimated supermassive black hole mass and low redshift. All models result in diffuse neutrino fluxes that are consistent with observations.
... Comparison of the 0.2-2 keV X-ray lightcurve evolution of J1331 (red markers) with other soft nuclear transients from quiescent galaxies (or those recently hosting low luminosity AGN). J1331 decays in X over longer timescales than QPEs (orange for eROQPE1; Arcodia et al. 2021), but still over much shorter timescales than previously reported TDEs in the literature, such as ASAS-SN 14li (grey, Bright et al. 2018), AT 2019azh decay phase (blue, Hinkle et al. 2020), AT 2019dsg (pink, Cannizzaro et al. 2021). The peak for J1331 was set to MJD=59592.9, ...
The ROSAT-selected tidal disruption event (TDE) candidate RX J133157.6-324319.7 (J1331), was detected in 1993 as a bright (0.2-2 keV flux of $(1.0 \pm 0.1) \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$), ultra-soft ($kT=0.11 \pm 0.03$ keV) X-ray flare from a quiescent galaxy ($z=0.05189$). During its fifth All-Sky survey (eRASS5) in 2022, SRG/eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2-2 keV flux of $(6.0 \pm 0.7) \times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$, with spectral properties ($kT=0.115 \pm 0.007$ keV) consistent with the ROSAT-observed flare $\sim$30 years earlier. In this work, we report on X-ray, UV, optical, and radio observations of this system. During a pointed XMM observation $\sim$17 days after the eRASS5 detection, J1331 was not detected in the 0.2-2 keV band, constraining the 0.2-2 keV flux to have decayed by a factor of $\gtrsim$40 over this period. Given the extremely low probability ($\sim5\times 10^{-6}$) of observing two independent full TDEs from the same galaxy over a 30 year period, we consider the variability seen in J1331 to be likely caused by two partial TDEs involving a star on an elliptical orbit around a black hole. J1331-like flares show faster rise and decay timescales ($\mathcal{O}(\mathrm{days})$) compared to standard TDE candidates, with neglible ongoing accretion at late times post-disruption between outbursts.
... Comparison of the 0.2-2 keV X-ray lightcurve evolution of J1331 (red markers) with other soft nuclear transients from quiescent galaxies (or those recently hosting low luminosity AGN). J1331 decays in X over longer timescales than QPEs (orange for eROQPE1;Arcodia et al. 2021), but still over much shorter timescales than previously reported TDEs in the literature, such as ASAS-SN 14li (grey,Bright et al. 2018), AT 2019azh decay phase (blue,Hinkle et al. 2020), AT 2019dsg (pink,Cannizzaro et al. 2021). The peak for J1331 was set to MJD=59592.9, ...
The ROSAT-selected tidal disruption event (TDE) candidate RX J133157.6-324319.7 (J1331), was detected in 1993 as a bright (0.2–2 keV flux of (1.0 ± 0.1) × 10−12 erg s−1 cm−2), ultra-soft (kT = 0.11 ± 0.03 keV) X-ray flare from a quiescent galaxy (z = 0.05189). During its fifth All-Sky survey (eRASS5) in 2022, SRG/eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2–2 keV flux of (6.0 ± 0.7) × 10−13 erg s−1 cm−2, with spectral properties (kT = 0.115 ± 0.007 keV) consistent with the ROSAT-observed flare ∼30 years earlier. In this work, we report on X-ray, UV, optical, and radio observations of this system. During a pointed XMM observation ∼17 days after the eRASS5 detection, J1331 was not detected in the 0.2–2 keV band, constraining the 0.2–2 keV flux to have decayed by a factor of ≳40 over this period. Given the extremely low probability (∼5 × 10−6) of observing two independent full TDEs from the same galaxy over a 30 year period, we consider the variability seen in J1331 to be likely caused by two partial TDEs involving a star on an elliptical orbit around a black hole. J1331-like flares show faster rise and decay timescales ($\mathcal {O}(\mathrm{days})$) compared to standard TDE candidates, with neglible ongoing accretion at late times post-disruption between outbursts.
... First we demonstrate an application of the generalized equipartition method to a well-observed event AT 2019dsg (Lee et al. 2020;Cannizzaro et al. 2021;Cendes et al. 2021;Stein et al. 2021;Mohan et al. 2022). It has been suggested that AT 2019dsg was associated by a high-energy neutrino (Stein et al. 2021). ...
The equipartition analysis yields estimates of the radius and energy of synchrotron self-absorbed radio sources. Here we generalize this method to relativistic off-axis viewed emitters. We find that the Lorentz factor $\Gamma$ and the viewing angle $\theta$ cannot be determined independently but become degenerate along a trajectory of minimal energy solutions. The solutions are divided into on-axis and off-axis branches with the former reproducing the classical analysis. A relativistic source viewed off-axis can be disguised as an apparent Newtonian one. Applying this method to radio observations of several tidal disruption events (TDEs), we find that the radio flare of AT 2018hyz which was observed a few years after the optical discovery could have been produced by a relativistic off-axis jet with kinetic energy of $\sim10^{53}\,\rm erg$ that was launched around the time of discovery.
... Some models consider emission from a relativistic jet, while others propose additional neutrino production scenarios e.g in a disk, disk corona or wind (see Hayasaki [39] for a review). In the case of AT2019dsg, radio observations confirmed long-lived non-thermal emission from the source [10,[40][41][42][43], but generally challenge the subset of models relying on the presence of an on-axis relativistic jet [34] in the standard leptonic radio emission model. ...
The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time X-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.
... Some models consider emission from a relativistic jet, while others propose additional neutrino production scenarios e.g in a disk, disk corona or wind (see Hayasaki [39] for a review). In the case of AT2019dsg, radio observations confirmed long-lived non-thermal emission from the source [10,[40][41][42][43], but generally challenge the subset of models relying on the presence of an on-axis relativistic jet [34] in the standard leptonic radio emission model. ...
The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time X-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.
... AT2019dsg was also detected at X-ray wavelengths by the X-Ray Telescope (XRT) [50], another telescope on board the Neil Gehrels Swift Observatory. It was further detected by the X-ray Multi-Mirror Mission (XMM)-Newton telescope [51], as well as Neutron star Interior Composition ExploreR (NICER) [52,53]. As can be seen in Figure 7, AT2019dsg was an initially-bright source which faded extremely rapidly over the course of observations. ...
Tidal Disruption Events (TDEs) occur when stars pass close to supermassive black holes, and have long been predicted to emit cosmic rays and neutrinos. Recently the TDE AT2109dsg was identified in spatial and temporal coincidence with high-energy neutrino IC191001A as part of the Zwicky Transient Facility (ZTF) neutrino follow-up program, providing the first direct observational evidence supporting these objects as multi-messenger sources. In these proceedings, I will place the recent results of our ZTF neutrino follow-up program into the broader context of developments in TDE and neutrino astronomy.
... AT2019dsg was also detected at X-ray wavelengths by the X-Ray Telescope (XRT) [50], another telescope on board the Neil Gehrels Swift Observatory. It was further detected by the X-ray Multi-Mirror Mission (XMM)-Newton telescope [51], as well as Neutron star Interior Composition ExploreR (NICER) [52,53]. As can be seen in Figure 7, AT2019dsg was an initially-bright source which faded extremely rapidly over the course of observations. ...
... This event was discovered as a bright UV/optical TDE on 9 April 2019 by the Zwicky Transient Facility (van Velzen et al. 2021). Follow-up observation revealed rapidly declining Xrays 40 days after the discovery (Cannizzaro et al. 2021;Stein et al. 2021) and radio emission lasting more than 500 days (Stein et al. 2021;Cendes et al. 2021;Mohan et al. 2021). In addition, IceCube reported detection of a high-energy neutrino in the same direction as the TDE (Stein et al. 2021), although the probability that this is actually associated with the TDE is not high. ...
The tidal disruption event, AT2019dsg, was observed across a broad range of electromagnetic-wavelengths from radio to X-rays, and it was possibly accompanied by a high-energy neutrino. We study the nature of the radio-emitting outflow by analyzing the synchrotron self-absorbed spectra in terms of the equipartition model. We find that the time evolution of the outflow radius can be interpreted as either free expansion or deceleration. If the former, the outflow was launched $\simeq$ 40 days before the optical peak; if the latter, the launch was $\simeq$ 10 days after the optical peak. In addition, the energy in the radio-emitting region increases over time. This second conclusion is most naturally interpreted by a scenario resembling the earliest stage of a supernova remnant: as more and more material is swept up, it is heated by the forward shock at the expense of the outflow's kinetic energy. Energy injection from an accreting BH cannot be completely excluded, but the injection rate is very different from the fallback luminosity, requiring further physical explanation. If the neutrino association is real, the scale of energy injection needed is much greater than for the radio emission, suggesting that the detected neutrino did not arise from the radio emitting region.
... Optically detected TDEs often go undetected in X-rays (Gezari et al. 2012), and vice versa. Although, there are events that have been found to emit in optical and X-rays (e.g., ASASSN-14li, ASASSN-15oi, AT 2019dsg, AT 2018fyk;Holoien et al. 2016;Gezari et al. 2017;Cannizzaro et al. 2021;Wevers et al. 2019, respectively). ...
We present results of our analysis of spectra of the host galaxies of the candidate Tidal Disruption Events (TDEs) XMMSL1 J111527.3+180638 and PTF09axc to determine the nature of these transients. We subtract the starlight component from the host galaxy spectra to determine the origin of the nuclear emission lines. Using a Baldwin-Phillips-Terlevich (BPT) diagram we conclude that the host galaxy of XMMSL1 J111527.3+180638 is classified as a Seyfert galaxy, suggesting this transient is likely to be caused by (extreme) variability in the active galactic nucleus. We find that the host of PTF09axc falls in the 'star-forming' region of the BPT-diagram, implying that the transient is a strong TDE candidate. For both galaxies we find a WISE-colour difference of $W1-W2<0.8$, which means there is no indication of a dusty torus and therefore an active galactic nucleus, seemingly contradicting our BPT finding for the host of XMMSL1 J111527.3+180638. We discuss possible reasons for the discrepant results obtained through the two methods.
... ASASSN-20hx was also observed using the X-ray timing instrument (XTI) onboard the Neutron star Interior Composition ExploreR (NICER: Gendreau et al. 2012), which is a external payload on the International Space Station. NICER offers high spectral (∼ 85 eV at 1 keV) and time resolution (∼ 100 ns) observations in the 0.2-12 keV energy range and has been used to observe a number of nuclear transients (Trakhtenbrot et al. 2019a;Pasham & Gendreau 2020;Pasham et al. 2020a,b;Hinkle et al. 2021b;Payne et al. 2021a;Ricci et al. 2021;Cannizzaro et al. 2021;Wevers et al. 2021). ASASSN-20hx was observed a total of 41 times between 2020 July 25 and 2020 November 01 (ObsIDs: 3573010101−3573014102, PI: Auchettl), for a total cumulative exposure of 67.6 ks. ...
We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from $-$30 to 275 days relative to peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite (TESS) data, we determine that the ANT began to brighten on 2020 June 23.3 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 29.5 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of $L = (3.15 \pm 0.04) \times 10^{43}$ erg s$^{-1}$. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to $L_{x} \sim 1.5 \times 10^{42}$ erg s$^{-1}$ and then slowly declined over time. The X-ray emission is well-fit by a power law with a photon index of $\Gamma \sim 2.3 - 2.6$. Both the optical and near infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events (TDEs) and active galactic nuclei (AGNs), it cannot be definitively classified with current data.
The multiwavelength data of nuclear transient AT2018dyk, initially discovered as a changing-look low-ionization nuclear emission-line region (LINER) galaxy, has been revisited by us and found being in agreement with a tidal disruption event (TDE) scenario. The optical light curve of AT2018dyk declines as a power-law form approximately with index -5/3 yet its X-ray emission lags behind the optical peak by ∼140 days, both of which are typical characteristics for TDEs. The X-ray spectra are softer than normal active galactic nuclei (AGNs) although they show a slight trend of hardening. Interestingly, its rising time scale belongs to the longest among TDEs while it is nicely consistent with the theoretical prediction from its relatively large supermassive black hole (SMBH) mass (∼107.38M⊙). Moreover, a prominent infrared echo with peak luminosity ∼7.4 × 1042 erg s−1 has been also detected in AT2018dyk, implying an unusually dusty subparsec nuclear environment in contrast with other TDEs. In our sample, LINERs share similar covering factors with AGNs, which indicates the existence of the dusty torus in these objects. Our work suggests that the nature of nuclear transients in LINERs needs to be carefully identified and their infrared echoes offer us a unique opportunity for exploring the environment of SMBHs at low accretion rate, which has been so far poorly explored but is crucial for understanding the SMBH activity.
The multiwavelength data of nuclear transient AT2018dyk, initially discovered as a changing-look low-ionization nuclear emission-line region (LINER) galaxy, has been revisited by us and found being in agreement with a tidal disruption event (TDE) scenario. The optical light curve of AT2018dyk declines as a power-law form approximately with index -5/3 yet its X-ray emission lags behind the optical peak by $\sim140$ days, both of which are typical characteristics for TDEs. The X-ray spectra are softer than normal active galactic nuclei (AGNs) although they show a slight trend of hardening. Interestingly, its rising time scale belongs to the longest among TDEs while it is nicely consistent with the theoretical prediction from its relatively large supermassive black hole (SMBH) mass ($\sim10^{7.38} M_{\odot}$). Moreover, a prominent infrared echo with peak luminosity $\sim7.4\times10^{42}~\text{erg}~\text{s}^{-1}$ has been also detected in AT2018dyk, implying an unusually dusty subparsec nuclear environment in contrast with other TDEs. In our sample, LINERs share similar covering factors with AGNs, which indicates the existence of the dusty torus in these objects. Our work suggests that the nature of nuclear transients in LINERs needs to be carefully identified and their infrared echoes offer us a unique opportunity for exploring the environment of SMBHs at low accretion rate, which has been so far poorly explored but is crucial for understanding the SMBH activity.
Tidal disruption events (TDEs) occur when a star gets torn apart by a supermassive black hole as it crosses its tidal radius. We present late-time optical and X-ray observations of the nuclear transient AT2019qiz, which showed the typical signs of an optical-UV transient class commonly believed to be TDEs. Optical spectra were obtained 428, 481 and 828 rest-frame days after optical lightcurve peak, and a UV/X-ray observation coincided with the later spectrum. The optical spectra show strong coronal emission lines, including [Fe VII], [Fe X], [Fe XI] and [Fe XIV]. The Fe lines rise and then fall, except [Fe XIV] which appears late and rises. We observe increasing flux of narrow Hα and Hβ and a decrease in broad Hα flux. The coronal lines have FWHMs ranging from ∼150 − 300km s−1, suggesting they originate from a region between the broad and narrow line emitting gas. Between the optical flare and late-time observation, the X-ray spectrum softens dramatically. The 0.3-1 keV X-ray flux increases by a factor of ∼50 while the hard X-ray flux decreases by a factor of ∼6. WISE fluxes also rose over the same period, indicating the presence of an infrared echo. With AT2017gge, AT2019qiz is one of two examples of a spectroscopically-confirmed optical-UV TDE showing delayed coronal line emission, supporting speculations that Extreme Coronal Line Emitters in quiescent galaxies can be echos of unobserved past TDEs. We argue that the coronal lines, narrow lines, and infrared emission arise from the illumination of pre-existing material likely related to either a previous TDE or AGN activity.
We present an expansion of FLEET, a machine-learning algorithm optimized to select transients that are most likely tidal disruption events (TDEs). FLEET is based on a random forest algorithm trained on both the light curves and host galaxy information of 4779 spectroscopically classified transients. We find that for transients with a probability of being a TDE, P (TDE) > 0.5, we can successfully recover TDEs with ≈40% completeness and ≈30% purity when using their first 20 days of photometry or a similar completeness and ≈50% purity when including 40 days of photometry, an improvement of almost 2 orders of magnitude compared to random selection. Alternatively, we can recover TDEs with a maximum purity of ≈80% and a completeness of ≈30% when considering only transients with P (TDE) > 0.8. We explore the use of FLEET for future time-domain surveys such as the Legacy Survey of Space and Time on the Vera C. Rubin Observatory (Rubin) and the Nancy Grace Roman Space Telescope (Roman). We estimate that ∼10 ⁴ well-observed TDEs could be discovered every year by Rubin and ∼200 TDEs by Roman. Finally, we run FLEET on the TDEs from our Rubin survey simulation and find that we can recover ∼30% of them at redshift z < 0.5 with P (TDE) > 0.5, or ∼3000 TDEs yr –1 that FLEET could uncover from the Rubin stream. We have demonstrated that we will be able to run FLEET on Rubin photometry as soon as this survey begins. FLEET is provided as an open source package on GitHub: https://github.com/gmzsebastian/FLEET .
The equipartition analysis yields estimates of the radius and energy of synchrotron self-absorbed radio sources. Here we generalize this method to relativistic off-axis viewed emitters. We find that the Lorentz factor Γ and the viewing angle θ cannot be determined independently but become degenerate along a trajectory of minimal energy solutions. The solutions are divided into on-axis and off-axis branches, with the former reproducing the classical analysis. A relativistic source viewed off-axis can be disguised as an apparent Newtonian one. Applying this method to radio observations of several tidal disruption events, we find that the radio flare of AT 2018hyz, which was observed a few years after the optical discovery, could have been produced by a relativistic off-axis jet with a kinetic energy of $\sim 10^{53}\, \rm erg$ that was launched around the time of discovery.
Winds can be launched in tidal disruption event (TDE). It has been proposed that the winds can interact with the cloud surrounding the black hole, produce bow shocks, accelerate electrons, and produce radio emission. We restudy the wind-cloud interaction model. We employ the properties of winds found by the radiation hydrodynamic simulations of super-Eddington circularized accretion flow in TDEs. We can calculate the peak radio emission frequency, the luminosity at the peak frequency, and their time-evolution based on the TDEs wind-cloud interaction model. We find that the model predicted peak radio emission frequency, the luminosity at peak frequency, and their time evolution can be well consistent with those in TDEs AT2019dsg and ASASSN-14li. This indicates that in these two radio TDEs, the wind-cloud interaction mechanism may be responsible for the radio emission.
AT 2019avd is a nuclear transient detected from infrared to soft X-rays, though its nature is yet unclear. The source has shown two consecutive flaring episodes in the optical and the infrared bands and its second flare was covered by X-ray monitoring programs. During this flare, the UVOT/Swift photometries revealed two plateaus: one observed after the peak and the other one appeared ∼240 days later. Meanwhile, our NICER and XRT/Swift campaigns show two declines in the X-ray emission, one during the first optical plateau and one 70–90 days after the optical/UV decline. The evidence suggests that the optical/UV could not have been primarily originated from X-ray reprocessing. Furthermore, we detected a timelag of ∼16–34 days between the optical and UV emission, which indicates the optical likely comes from UV reprocessing by a gas at a distance of 0.01 −0.03 pc. We also report the first VLA and VLBA detection of this source at different frequencies and different stages of the second flare. The information obtained in the radio band – namely a steep and a late-time inverted radio spectrum, a high brightness temperature and a radio-loud state at late times – together with the multiwavelength properties of AT 2019avd suggests the launching and evolution of outflows such as disc winds or jets. In conclusion, we propose that after the ignition of black hole activity in the first flare, a super-Eddington flaring accretion disc formed and settled to a sub-Eddington state by the end of the second flare, associated with a compact radio outflow.
We perform a comprehensive analysis of a population of 19 X-ray bright tidal disruption events (TDEs), fitting their X-ray spectra with a new, physically self consistent, relativistic accretion disc model. Not all of the TDEs inhabit regions of parameter space where the model is valid, or have sufficient data for a detailed analysis, and physically interpretable parameters for a sub-sample of 11 TDEs are determined. These sources have thermal (power-law free) X-ray spectra. The radial sizes measured from these spectra lie at values consistent with the inner-most stable circular orbit of black holes with masses given by the MBH − σ relationship, and can be used as an independent measurement of MBH. The bolometric disc luminosity can also be inferred from X-ray data. All of the TDEs have luminosities which are sub-Eddington (Lbol, disc ≲ Ledd), and larger than the typical hard-state transitional luminosity of X-ray binary discs (Lbol, disc ≳ 0.01Ledd). The peak bolometric luminosity is found to be linearly correlated with the MBH − σ mass. The TDE X-ray-to-bolometric correction can reach values up to ∼100, and grows exponentially at late times, resolving the missing energy problem. We show that the peak disc luminosities of some TDEs are smaller than their observed optical luminosities, implying that not all of the early time optical emission can be sourced from reprocessed disc emission. Our results are supportive of the hypothesis that thermal X-ray bright TDEs are in accretion states analogous to the ‘soft’ accretion state of X-ray binaries, and that black hole accretion processes are scale (mass) invariant.
Context. Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive black holes at the centers of galaxies – nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line intensities, often referred to as changing-look AGN (CLAGN). Given the physical and observational similarities, the interpretation and distinction of nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of making progress in the field is the lack of well-sampled data of long-lived nuclear outbursts in AGN.
Aims. Here, we study PS16dtm, a nuclear transient in a Narrow Line Seyfert 1 (NLSy1) galaxy, which has been proposed to be a TDE candidate. Our aim is to study the spectroscopic and photometric properties of PS16dtm, in order to better understand the outbursts originating in NLSy1 galaxies.
Methods. Our extensive multiwavelength follow-up that spans around 2000 days includes photometry and spectroscopy in the UV/optical, as well as mid-infrared (MIR) and X-ray observations. Furthermore, we improved an existing semiempirical model in order to reproduce the spectra and study the evolution of the spectral lines.
Results. The UV/optical light curve shows a double peak at ∼50 and ∼100 days after the first detection, and it declines and flattens afterward, reaching preoutburst levels after 2000 days of monitoring. The MIR light curve rises almost simultaneously with the optical, but unlike the UV/optical which is approaching the preoutburst levels in the last epochs of our observations, the MIR emission is still rising at the time of writing. The optical spectra show broad Balmer features and the strongest broad Fe II emission ever detected in a nuclear transient. This broad Fe II emission was not present in the archival preoutburst spectrum and almost completely disappeared +1868 days after the outburst. We found that the majority of the flux of the broad Balmer and Fe II lines is produced by photoionization. We detect only weak X-ray emission in the 0.5−8 keV band at the location of PS16dtm, at +848, +1130, and +1429 days past the outburst. This means that the X-ray emission continues to be lower by at least an order of magnitude, compared to archival, preoutburst measurements.
Conclusions. We confirm that the observed properties of PS16dtm are difficult to reconcile with normal AGN variability. The TDE scenario continues to be a plausible explanation for the observed properties, even though PS16dtm shows differences compared to TDE in quiescent galaxies. We suggest that this event is part of a growing sample of TDEs that show broad Balmer line profiles and Fe II complexes. We argue that the extreme variability seen in the AGN host due to PS16dtm may have easily been misclassified as a CLAGN, especially if the rising part of the light curve had been missed. This implies that some changing look episodes in AGN may be triggered by TDEs. Imaging and spectroscopic data of AGN with good sampling are needed to enable testing of possible physical mechanisms behind the extreme variability in AGN.
We present an expansion of FLEET, a machine learning algorithm optimized to select transients that are most likely to be tidal disruption events (TDEs). FLEET is based on a random forest algorithm trained on the light curves and host galaxy information of 4,779 spectroscopically classified transients. For transients with a probability of being a TDE, \ptde$>0.5$, we can successfully recover TDEs with a $\approx40$\% completeness and a $\approx30$\% purity when using the first 20 days of photometry, or a similar completeness and $\approx50$\% purity when including 40 days of photometry. We find that the most relevant features for differentiating TDEs from other transients are the normalized host separation, and the light curve $(g-r)$ color during peak. Additionally, we use FLEET to produce a list of the 39 most likely TDE candidates discovered by the Zwicky Transient Facility that remain currently unclassified. We explore the use of FLEET for the Legacy Survey of Space and Time on the Vera C. Rubin Observatory (\textit{Rubin}) and the \textit{Nancy Grace Roman Space Telescope} (\textit{Roman}). We simulate the \textit{Rubin} and \textit{Roman} survey strategies and estimate that $\sim 10^4$ TDEs could be discovered every year by \textit{Rubin}, and $\sim200$ TDEs per year by \textit{Roman}. Finally, we run FLEET on the TDEs in our \textit{Rubin} survey simulation and find that we can recover $\sim 30$\% of those at a redshift $z <0.5$ with \ptde$>0.5$. This translates to $\sim3,000$ TDEs per year that FLEET could uncover from \textit{Rubin}. FLEET is provided as a open source package on GitHub https://github.com/gmzsebastian/FLEET
With the sample of observed tidal disruption events (TDEs) now reaching several tens, distinct spectroscopic classes have emerged: TDEs with only hydrogen lines (TDE-H), only helium lines (TDE-He), or hydrogen in combination with He ii and often N iii/O iii (TDE-H+He). Here we model the light curves of 32 optically-bright TDEs using the Modular Open Source Fitter for Transients (mosfit) to estimate physical and orbital properties, and look for statistical differences between the spectroscopic classes. For all types, we find a shallow distribution of star masses, compared to a typical initial mass function, between ∼0.1–1 M⊙, and no TDEs with very deep encounters. Our main result is that TDE-H events appear to come from less complete disruptions (and possibly lower SMBH masses) than TDE-H+He, with TDE-He events fully disrupted. We also find that TDE-H events have more extended photospheres, in agreement with recent literature, and argue that this could be a consequence of differences in the self-intersection radii of the debris streams. Finally, we identify an approximately linear correlation between black hole mass and radiative efficiency. We suggest that TDE-H may be powered by collision-induced outflows at relatively large radii, while TDE-H+He could result from prompt accretion disks, formed more efficiently in closer encounters around more massive SMBHs.
Aims. The modelling of spectroscopic observations of tidal disruption events (TDEs) to date suggests that the newly formed accretion disks are mostly quasi-circular. In this work we study the transient event AT 2020zso, hosted by an active galactic nucleus (AGN; as inferred from narrow emission line diagnostics), with the aim of characterising the properties of its newly formed accretion flow.
Methods. We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations and spectral line content and evolution. We identify transient, double-peaked Bowen (N III ), He I , He II, and H α emission lines. We model medium-resolution optical spectroscopy of the He II (after careful de-blending of the N III contribution) and H α lines during the rise, peak, and early decline of the light curve using relativistic, elliptical accretion disk models.
Results. We find that the spectral evolution before the peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around the peak, the envelope reaches its maximum extent (approximately 10 ¹⁵ cm, or ∼3000–6000 gravitational radii for an inferred black hole mass of 5−10 × 10 ⁵ M ⊙ ) and becomes optically thin. The H α and He II emission lines at and after the peak can be reproduced with a highly inclined ( i = 85 ± 5 degrees), highly elliptical ( e = 0.97 ± 0.01), and relatively compact ( R in = several 100 R g and R out = several 1000 R g ) accretion disk.
Conclusions. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs, where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary supermassive black hole hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination). We constrain the disk alignment timescale to > 15 days in AT2020zso, which rules out high black hole spin values ( a < 0.8) for M BH ∼ 10 ⁶ M ⊙ and disk viscosity α ≳ 0.1.
Energetic nuclear outbursts have been discovered in luminous and ultraluminous infrared galaxies (U/LIRGs) at unexpectedly high rates. To investigate this population of transients, we performed a search in mid-IR data from the Wide-field Infrared Survey Explorer (WISE) satellite and its NEOWISE survey to detect and characterise luminous and smoothly evolving transients in a sample of 215 U/LIRGs. We report three new transients, all with Δ L > 10 ⁴³ erg s ⁻¹ , in addition to two previously known cases. Their host galaxies are all part of major galaxy mergers, and through radiative transfer model fitting we find that all have a significant contribution from an active galactic nucleus (AGN). We characterised the transients through measurements of their luminosities and resulting energetics, all of which are between 10 50.9 erg and 10 52.2 erg. The IR emission of the five transients was found to be consistent with re-radiation by the hot dust of emission at shorter wavelengths, presumably originating from an accretion event, onto the supermassive black hole. The corresponding transient rate of (1.6–4.6) × 10 ⁻³ yr ⁻¹ galaxy ⁻¹ is over an order of magnitude higher than the rate of large amplitude flares shown by AGN in the optical. We suggest that the observed transients are part of a dust-obscured population of tidal disruption events (TDEs) that have remained out of the reach of optical surveys due to the obscuring dust. In one case, this is supported by our radio observations. We also discuss other plausible explanations. The observed rate of events is significantly higher than optical TDE rates, which can be expected in U/LIRG hosts undergoing a major galaxy merger with increased stellar densities in the nuclear regions. Continued searches for such transients and their multi-wavelength follow-up is required to constrain their rate and nature.
A tidal disruption event (TDE), AT2019dsg, was observed to be associated with a PeV neutrino event, IceCube-191001A, lagging the optical outburst by a half year. It is known that TDEs may generate ultrafast outflows. If the TDE occurs in a cloudy environment, the outflow-cloud interactions may form shock waves which generate accelerated protons and hence delayed neutrinos from hadronic interactions in clouds. Here we investigate the neutrino production in AT2019dsg by examining the TDE outflow-cloud interaction model. We find that, for an outflow with a velocity of 0.07c and a kinetic luminosity of $10^{45}\rm erg\ s^{-1}$, protons may be accelerated up to ∼ 60 PeV by the bow shocks, and generate PeV neutrinos by interactions with clouds. The predicted neutrino number in this model depends on the uncertainties of model parameters and in order to match the observations, some challenging values of parameters have been involved. The PeV neutrino event number can be ∼4 × 10−3 for a hard proton index Γ = 1.5.
We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from −30 to 275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of L = (3.15 ± 0.04) × 10 ⁴³ erg s ⁻¹ . The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to L x ∼ 1.5 × 10 ⁴² erg s ⁻¹ and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of Γ ∼ 2.3–2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data.
The tidal disruption event AT2019dsg was observed from radio to X-rays and was possibly accompanied by a high-energy neutrino. Previous interpretations have focused on continued injection by a central engine as the source of energy for radio emission. We show that continuous energy injection is unnecessary; the radio data can be explained by a single ejection of plasma that supplies all the energy needed. To support this assertion, we analyze the synchrotron self-absorbed spectra in terms of the equipartition model. Similar to previous analyses, we find that the energy in the radio-emitting region increases approximately ∝t0.7 and the lengthscale of this region grows ∝t at a rate ≃ 0.06 c. This event resembles the earliest stage of a supernova remnant: because the ejected mass is much greater than the shocked external mass, its velocity remains unchanged, while the energy in shocked gas grows with time. The radio-emitting material gains energy from the outflow, not continuing energy injection by the central object. Although energy injection from an accreting BH cannot be completely excluded, the energy injection rate is very different from the fallback luminosity, and maintaining constant outflow velocity requires fine-tuning demanding further physical explanation. If the neutrino association is real, the energy injection needed is much greater than for the radio emission, suggesting that the detected neutrino did not arise from the radio-emitting region.
With the sample of observed tidal disruption events (TDEs) now reaching several tens, distinct spectroscopic classes have emerged: TDEs with only hydrogen lines (TDE-H), only helium lines (TDE-He), or hydrogen in combination with He II and often N III/O III (TDE-H+He). Here we model the light curves of 32 optically-bright TDEs using the Modular Open Source Fitter for Transients (MOSFiT) to estimate physical and orbital properties, and look for statistical differences between the spectroscopic classes. For all types, we find a shallow distribution of star masses, compared to a typical initial mass function, between $\sim 0.1-1$ M$_\odot$, and no TDEs with very deep ($\beta \gg 1$) encounters. Our main result is that TDE-H events appear to come from less complete disruptions (and possibly lower SMBH masses) than TDE-H+He, with TDE-He events fully disrupted. We also find that TDE-H events have more extended photospheres, in agreement with recent literature, and argue that this could be a consequence of differences in the self-intersection radii of the debris streams. Finally, we identify an approximately linear correlation between black hole mass and radiative efficiency. We suggest that TDE-H may be powered by collision-induced outflows at relatively large radii, while TDE-H+He could result from prompt accretion disks, formed more efficiently in closer encounters around more massive SMBHs.
Spectroscopically, tidal disruption events (TDEs) are characterized by broad (∼10 ⁴ km s ⁻¹ ) emission lines and show a large diversity as well as different line profiles. After carefully and consistently performing a series of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical and UV TDEs. We study a number of emission lines prominent among TDEs including Hydrogen, Helium, and Bowen lines and we quantify their evolution with time in terms of line luminosities, velocity widths, and velocity offsets. We report a time lag between the peaks of the optical light curves and the peak luminosity of H α spanning between ∼7 and 45 days. If interpreted as light echoes, these lags correspond to distances of ∼2 − 12 × 10 ¹⁶ cm, which are one to two orders of magnitudes larger than the estimated blackbody radii ( R BB ) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of the He I 5876 Å line, which are smaller than the ones of H α for H TDEs and similar or larger for N III Bowen TDEs. We report that N III Bowen TDEs have lower H α velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii ( R BB ) and temperatures ( T BB ). We find a linear relationship between H α luminosity and the R BB ( L line ∝ R BB ) and potentially an inverse power-law relation with T BB ( L line ∝ T BB −β ), leading to weaker H α emission for T BB ≥ 25 000 K. The He II /He I ratio becomes large at the same temperatures, possibly pointing to an ionization effect. The He II /H α ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties can potentially be attributed to viewing angle effects.
We propose a model to explain the time delay between the peak of the optical and X-ray luminosity, Δt O-X hereafter, in UV/optically selected tidal disruption events (TDEs). The following picture explains the observed Δt O-X in several TDEs as a consequence of the circularization and disk accretion processes as long as the sub-Eddington accretion. At the beginning of the circularization, the fallback debris is thermalized by the self-crossing shock caused by relativistic precession, providing the peak optical emission. During the circularization process, the mass fallback rate decreases with time to form a ring around the supermassive black hole (SMBH). The formation timescale corresponds to the circularization timescale of the most tightly bound debris, which is less than a year to several decades, depending mostly on the penetration factor, the circularization efficiency, and the black hole mass. The ring will subsequently evolve viscously over the viscous diffusion time. We find that it accretes onto the SMBH on a fraction of the viscous timescale, which is 2 years for the typical given parameters, leading to X-ray emission at late times. The resultant Δt O-X is given by the sum of the circularization timescale and the accretion timescale and significantly decreases with increasing penetration factor to several to ∼10 yr typically. Since the X-ray luminosity substantially decreases as the viewing angle between the normal to the disk plane and line of sight increases from 0° to 90°, a low late-time X-ray luminosity can be explained by an edge-on view. We also discuss the super-Eddington accretion scenario, where Δt O-X is dominated by the circularization timescale. © 2021. The American Astronomical Society. All rights reserved.
We present results of our analysis of spectra of the host galaxies of the candidate tidal disruption events (TDEs) XMMSL1 J111527.3+180638 and PTF09axc to determine the nature of these transients. We subtract the starlight component from the host galaxy spectra to determine the origin of the nuclear emission lines. Using a Baldwin–Phillips–Terlevich (BPT) diagram, we conclude that the host galaxy of XMMSL1 J111527.3+180638 is classified as a Seyfert galaxy, suggesting this transient is likely to be caused by (extreme) variability in the active galactic nucleus. We find that the host of PTF09axc falls in the ‘star-forming’ region of the BPT diagram, implying that the transient is a strong TDE candidate. For both galaxies, we find a WISE colour difference of W1 − W2 < 0.8, which means there is no indication of a dusty torus and therefore an active galactic nucleus, seemingly contradicting our BPT finding for the host of XMMSL1 J111527.3+180638. We discuss possible reasons for the discrepant results obtained through these two methods.
At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≈106 M⊙, disrupting a star of ≈1 M⊙. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent with a photosphere expanding at constant velocity (≳2000 km s−1), and a line-forming region producing initially blueshifted H and He ii profiles with v = 3000–10 000 km s−1. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission – the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N iii) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≈1041 erg s−1. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.
We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) candidate AT 2018fyk/ASASSN–18ul discovered by ASAS–SN. The Swift light curve is atypical for a TDE, entering a plateau after ∼40 d of decline from peak. After 80 d the UV/optical light curve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He, and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high-ionization (O iii) and low-ionization (Fe ii) lines, which are visible for ∼45 d. We similarly identify Fe ii lines in optical spectra of ASASSN–15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies, and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe ii lines can be associated with the formation of a compact accretion disc, as the emergence of low-ionization emission lines requires optically thick, high-density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a two-component light curve is very similar to that seen in the TDE candidate ASASSN–15lh, and is a natural consequence of a relativistic orbital pericentre.
After a star has been tidally disrupted by a black hole, the debris forms an elongated stream. We start by studying the evolution of this gas before its bound part returns to the original stellar pericenter. While the axial motion is entirely ballistic, the transverse directions of the stream are usually thinner due to the confining effects of self-gravity. This basic picture may also be influenced by additional physical effects such as clump formation, hydrogen recombination, magnetic fields and the interaction with the ambient medium. We then examine the fate of this stream when it comes back to the vicinity of the black hole to form an accretion flow. Despite recent progress, the hydrodynamics of this phase remains uncertain due to computational limitations that have so far prevented us from performing a fully self-consistent simulation. Most of the initial energy dissipation appears to be provided by a self-crossing shock that results from an intersection of the stream with itself. The debris evolution during this collision depends on relativistic apsidal precession, expansion of the stream from pericenter, and nodal precession induced by the black hole spin. Although the combined influence of these effects is not fully understood, current works suggest that this interaction is typically too weak to significantly circularize the trajectories, with its main consequence being an expansion of the shocked gas. Global simulations of disc formation using simplified initial conditions find that the debris experiences additional collisions that cause its orbits to become more circular until eventually settling into a thick structure. These works suggest that this process completes faster for more relativistic encounters due to stronger shocks. However, important aspects still remain to be understood at the time of writing, due to numerical challenges and the complexity of this process.
Radio observations of tidal disruption events (TDEs) probe material ejected by the disruption of stars by supermassive black holes (SMBHs), uniquely tracing the formation and evolution of jets and outflows, revealing details of the disruption hydrodynamics, and illuminating the environments around previously-dormant SMBHs. To date, observations reveal a surprisingly diverse population. A small fraction of TDEs (at most a few percent) have been observed to produce radio-luminous mildly relativistic jets. The remainder of the population are radio quiet, producing less luminous jets, non-relativistic outflows or, possibly, no radio emission at all. Here, we review the radio observations that have been made of TDEs to date and discuss possible explanations for their properties, focusing on detected sources and, in particular, on the two best-studied events: Sw J1644+57 and ASASSN-14li. We also discuss what we have learned about the host galaxies of TDEs from radio observations and review constraints on the rates of bright and faint radio outflows in TDEs. Upcoming X-ray, optical, near-IR, and radio surveys will greatly expand the sample of TDEs, and technological advances open the exciting possibility of discovering a sample of TDEs in the radio band unbiased by host galaxy extinction.
An expanding radio jet from a destroyed star
If a star gets too close to a supermassive black hole, it gets ripped apart in a tidal disruption event (TDE). Mattila et al. discovered a transient source in the merging galaxy pair Arp 299, which they interpret as a TDE. The optical light is hidden by dust, but the TDE generated copious infrared emission. Radio observations reveal that a relativistic jet was produced as material fell onto the black hole, with the jet expanding over several years. The results elucidate how jets form around supermassive black holes and suggest that many TDEs may be missed by optical surveys.
Science , this issue p. 482
The mass of the central black hole in a galaxy that hosted a tidal disruption event (TDE) is an important parameter in understanding its energetics and dynamics. We present the first homogeneously measured black hole masses of a complete sample of 12 optically/UV selected TDE host galaxies (down to $g_{host}$$\leq$22 mag and $z$=0.37) in the Northern sky. The mass estimates are based on velocity dispersion measurements, performed on late time optical spectroscopic observations. We find black hole masses in the range 3$\times$10$^5$ M$_{\odot}$$\leq$M$_{\rm BH}$$\leq$2$\times$10$^7$ M$_{\odot}$. The TDE host galaxy sample is dominated by low mass black holes ($\sim$10$^6$ M$_{\odot}$), as expected from theoretical predictions. The blackbody peak luminosity of TDEs with M$_{\rm BH}$$\leq$10$^{7.1}$ M$_{\odot}$ is consistent with the Eddington limit of the SMBH, whereas the two TDEs with M$_{\rm BH}$$\geq$10$^{7.1}$ M$_{\odot}$ have peak luminosities below their SMBH Eddington luminosity, in line with the theoretical expectation that the fallback rate for M$_{\rm BH}$$\geq$10$^{7.1}$ M$_{\odot}$ is sub-Eddington. In addition, our observations suggest that TDEs around lower mass black holes evolve faster. These findings corroborate the standard TDE picture in 10$^6$ M$_{\odot}$ black holes. Our results imply an increased tension between observational and theoretical TDE rates. By comparing the blackbody emission radius with theoretical predictions, we conclude that the optical/UV emission is produced in a region consistent with the stream self-intersection radius of shallow encounters, ruling out a compact accretion disk as the direct origin of the blackbody radiation at peak brightness.
Tidal Disruption Events (TDEs) are transient events observed when a star
passes close enough to a supermassive black hole to be tidally destroyed. Many
TDE candidates have been discovered in host galaxies whose spectra have weak or
no line emission yet strong Balmer line absorption, indicating a period of
intense star formation that has recently ended. As such, TDE host galaxies fall
into the rare class of quiescent Balmer-strong galaxies. Here, we quantify the
fraction of galaxies in the Sloan Digital Sky Survey (SDSS) with spectral
properties like those of TDE hosts, determining the extent to which TDEs are
over-represented in such galaxies. Galaxies whose spectra have Balmer
absorption H$\delta_{\rm A}$ $-$ $\sigma$(H$\delta_{\rm A}$) $>$ 4 \AA\ (where
$\sigma$(H$\delta_{\rm A}$) is the error in the Lick H$\delta_{\rm A}$ index)
and H$\alpha$ emission EW $<$ $3$ \AA\ have had a strong starburst in the last
$\sim$Gyr. They represent 0.2% of the local galaxy population, yet host 3 of 8
(37.5%) optical/UV-selected TDE candidates. A broader cut, H$\delta_{\rm A} >$
1.31 \AA\ and H$\alpha$ EW $<$ $3$ \AA, nets only 2.3% of SDSS galaxies, but 6
of 8 (75%) optical/UV TDE hosts. Thus, quiescent Balmer-strong galaxies are
over-represented among the TDE hosts by a factor of 33-190. The
high-energy-selected TDE Swift J1644 also lies in a galaxy with strong Balmer
lines and weak H$\alpha$ emission, implying a $>80\times$ enhancement in such
hosts and providing an observational link between the $\gamma$/X-ray-bright and
optical/UV-bright TDE classes.
A tidal disruption event occurs when a star wanders close enough to a black hole to be disrupted by its tidal force. The debris
of a tidally disrupted star are expected to form an accretion disc around the supermassive black hole. The light curves of
these events sometimes show a quasi-periodic modulation of the flux that can be associated with the precession of the accretion
disc due to the Lense–Thirring (‘frame-dragging’) effect. Since the initial star orbit is in general inclined with respect
to the black hole spin, this misalignment combined with the Lense–Thirring effect leads to a warp in the disc. In this paper,
we provide a simple model of the system composed by a thick and narrow accretion disc surrounding a spinning supermassive
black hole, with the aim to: (a) compute the expected precession period as a function of the system parameters, (b) discuss
the conditions that have to be satisfied in order to have rigid precession, (c) investigate the alignment process, highlighting
how different mechanisms play a role leading the disc and the black hole angular momenta into alignment.
We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the centre
of PGC 043234 (d ≃ 90 Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of L ≃ 1044 erg s−1 and a total integrated energy of E ≃ 7 × 1050 erg radiated over the ∼6 months of observations presented. The UV/optical emission of the source is well fitted by a blackbody
with roughly constant temperature of T ∼ 35 000 K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is
broadly consistent with an exponential decline, $L\propto \text{e}^{-t/t_0}$, with t0 ≃ 60 d. ASASSN-14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining
at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission
as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN-14li and ASASSN-14ae to estimate
the TDE rate implied by ASAS-SN, finding an average rate of r ≃ 4.1 × 10−5 yr−1 per galaxy with a 90 per cent confidence interval of (2.2–17.0) × 10−5 yr−1 per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of
other surveys.
Context. The Public European Southern Observatory Spectroscopic Survey of Transient Objects (PESSTO) began as a public spectroscopic survey in April 2012. PESSTO classifies transients from publicly available sources and wide-field surveys, and selects science targets for detailed spectroscopic and photometric follow-up. PESSTO runs for nine months of the year, January - April and August - December inclusive, and typically has allocations of 10 nights per month. Aims. We describe the data reduction strategy and data products that are publicly available through the ESO archive as the Spectroscopic Survey data release 1 (SSDR1). Methods. PESSTO uses the New Technology Telescope with the instruments EFOSC2 and SOFI to provide optical and NIR spectroscopy and imaging. We target supernovae and optical transients brighter than 20.5(m) for classification. Science targets are selected for follow-up based on the PESSTO science goal of extending knowledge of the extremes of the supernova population. We use standard EFOSC2 set-ups providing spectra with resolutions of 13-18 angstrom between 3345-9995 angstrom. A subset of the brighter science targets are selected for SOFI spectroscopy with the blue and red grisms (0.935-2.53 mu m and resolutions 23-33 angstrom) and imaging with broadband JHK(s) filters. Results. This first data release (SSDR1) contains flux calibrated spectra from the first year (April 2012-2013). A total of 221 confirmed supernovae were classified, and we released calibrated optical spectra and classifications publicly within 24 h of the data being taken (via WISeREP). The data in SSDR1 replace those released spectra. They have more reliable and quantifiable flux calibrations, correction for telluric absorption, and are made available in standard ESO Phase 3 formats. We estimate the absolute accuracy of the flux calibrations for EFOSC2 across the whole survey in SSDR1 to be typically similar to 15%, although a number of spectra will have less reliable absolute flux calibration because of weather and slit losses. Acquisition images for each spectrum are available which, in principle, can allow the user to refine the absolute flux calibration. The standard NIR reduction process does not produce high accuracy absolute spectrophotometry but synthetic photometry with accompanying JHK(s) imaging can improve this. Whenever possible, reduced SOFI images are provided to allow this. Conclusions. Future data releases will focus on improving the automated flux calibration of the data products. The rapid turnaround between discovery and classification and access to reliable pipeline processed data products has allowed early science papers in the first few months of the survey.
Context: Absorption by molecules in the Earth's atmosphere strongly affects
ground-based astronomical observations. The resulting absorption line strength
and shape depend on the highly variable physical state of the atmosphere, i.e.
pressure, temperature, and mixing ratio of the different molecules involved.
Usually, supplementary observations of so-called telluric standard stars (TSS)
are needed to correct for this effect, which is expensive in terms of telescope
time. We have developed the software package molecfit to provide synthetic
transmission spectra based on parameters obtained by fitting narrow ranges of
the observed spectra of scientific objects. These spectra are calculated by
means of the radiative transfer code LBLRTM and an atmospheric model. In this
way, the telluric absorption correction for suitable objects can be performed
without any additional calibration observations of TSS. Aims: We evaluate the
quality of the telluric absorption correction using molecfit with a set of
archival ESO-VLT X-Shooter visible and near-infrared spectra. Methods: Thanks
to the wavelength coverage from the U to the K band, X-Shooter is well suited
to investigate the quality of the telluric absorption correction with respect
to the observing conditions, the instrumental set-up, input parameters of the
code, the signal-to-noise of the input spectrum, and the atmospheric profiles.
These investigations are based on two figures of merit, I_off and I_res, that
describe the systematic offsets and the remaining small-scale residuals of the
corrections. We also compare the quality of the telluric absorption correction
achieved with moelcfit to the classical method based on a telluric standard
star. (Abridged)
Context: The interaction of the light from astronomical objects with the
constituents of the Earth's atmosphere leads to the formation of telluric
absorption lines in ground-based collected spectra. Correcting for these lines,
mostly affecting the red and infrared region of the spectrum, usually relies on
observations of specific stars obtained close in time and airmass to the
science targets, therefore using precious observing time. Aims: We present
molecfit, a tool for correcting for telluric absorption lines based on
synthetic modelling of the Earth's atmospheric transmission. Molecfit is
versatile and can be used with data obtained with various ground-based
telescopes and instruments. Methods: Molecfit combines a publicly available
radiative transfer code, a molecular line database, atmospheric profiles, and
various kernels to model the instrument line spread function. The atmospheric
profiles are created by merging a standard atmospheric profile representative
of a given observatory's climate, of local meteorological data, and of
dynamically retrieved altitude profiles for temperature, pressure, and
humidity. We discuss the various ingredients of the method, its applicability,
and its limitations. We also show examples of telluric line correction on
spectra obtained with a suite of ESO Very Large Telescope (VLT) instruments.
Results: Compared to previous similar tools, molecfit takes the best results
for temperature, pressure, and humidity in the atmosphere above the observatory
into account. As a result, the standard deviation of the residuals after
correction of unsaturated telluric lines is frequently better than 2% of the
continuum. Conclusion: Molecfit is able to accurately model and correct for
telluric lines over a broad range of wavelengths and spectral resolutions.
(Abridged)
We study the circularization of tidally disrupted stars on bound orbits
around spinning supermassive black holes by performing three-dimensional
smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our
simulations reveal that debris circularization depends sensitively on the
efficiency of radiative cooling. There are two stages in debris circularization
if radiative cooling is inefficient: first, the stellar debris streams
self-intersect due to relativistic apsidal precession; shocks at the
intersection points thermalize orbital energy and the debris forms a
geometrically thick, ring-like structure around the black hole. The ring
rapidly spreads via viscous diffusion, leading to the formation of a
geometrically thick accretion disk. In contrast, if radiative cooling is
efficient, the stellar debris circularizes due to self-intersection shocks and
forms a geometrically thin ring-like structure. In this case, the dissipated
energy can be emitted during debris circularization as a precursor to the
subsequent tidal disruption flare. The possible radiated energy is up to
~2*10^{52} erg for a 1 Msun star orbiting a 10^6 Msun black hole. We also find
that a retrograde (prograde) black hole spin causes the shock-induced
circularization timescale to be shorter (longer) than that of a non-spinning
black hole in both cooling cases. The circularization timescale is remarkably
long in the radiatively efficient cooling case, and is also sensitive to black
hole spin. Specifically, Lense-Thirring torques cause dynamically important
nodal precession, which significantly delays debris circularization. On the
other hand, nodal precession is too slow to produce observable signatures in
the radiatively inefficient case. We also discuss the relationship between our
simulations and the parabolic TDEs that are characteristic of most stellar
tidal disruptions.
We model the non-thermal transient Swift J1644+57 as resulting from a
relativistic jet powered by the accretion of a tidally-disrupted star onto a
super-massive black hole. Accompanying synchrotron radio emission is produced
by the shock interaction between the jet and the dense circumnuclear medium,
similar to a gamma-ray burst afterglow. An open mystery, however, is the origin
of the late-time radio rebrightening, which occurred well after the peak of the
jetted X-ray emission. Here, we systematically explore several proposed
explanations for this behavior by means of multi-dimensional hydrodynamic
simulations coupled to a self-consistent radiative transfer calculation of the
synchrotron emission. Our main conclusion is that the radio afterglow of Swift
J1644+57 is not naturally explained by a jet with a one-dimensional top-hat
angular structure. However, a more complex angular structure comprised of an
ultra-relativistic core (Lorentz factor $\Gamma \sim 10$) surrounded by a
slower ($\Gamma \sim $ 2) sheath provides a reasonable fit to the data. Such a
geometry could result from the radial structure of the super-Eddington
accretion flow or as the result of jet precession. The total kinetic energy of
the ejecta that we infer of $\sim$ few $10^{53}\,$erg requires a highly
efficient jet launching mechanism. Our jet model providing the best fit to the
light curve of the on-axis event Swift J1644+57 is used to predict the radio
light curves for off-axis viewing angles. Implications for the presence of
relativistic jets from TDEs detected via their thermal disk emission, as well
as the prospects for detecting orphan TDE afterglows with upcoming wide-field
radio surveys and resolving the jet structure with long baseline
interferometry, are discussed.
We present the results of a Palomar Transient Factory (PTF) archival search
for blue transients which lie in the magnitude range between "normal"
core-collapse and superluminous supernovae (i.e. with
$-21\,{\leq}M_{R\,(peak)}\,{\leq}-19$). Of the six events found after excluding
all interacting Type IIn and Ia-CSM supernovae, three (PTF09ge, 09axc and
09djl) are coincident with the centers of their hosts, one (10iam) is offset
from the center, and for two (10nuj and 11glr) a precise offset can not be
determined. All the central events are similar photometrically to the He-rich
tidal disruption candidate PS1-10jh. Spectroscopically, PTF09ge is also
He-rich, while PTF09axc and 09djl display broad hydrogen features around peak
magnitude. All three central events are in low star-formation hosts, two of
which are E+A galaxies. Our spectrum of the host of PS1-10jh displays similar
properties. PTF10iam, the one offset event, is different photometrically and
spectroscopically from the central events and its host displays a higher star
formation rate. Finding no obvious evidence for AGN activity, we conclude that
the three central transients likely arise from the tidal disruption of a star
by a super-massive black hole. We compare the spectra of these events to TDE
candidates from the literature and to the recent ASASSN-14ae, and find that all
of these objects can be unified on a continuous scale of spectral properties.
The accumulated evidence of this expanded sample strongly supports a tidal
disruption origin for this class of nuclear transients.
We study the kinematic properties of the ambient ionized ISM and ionized gas
outflows in a large and representative sample of local luminous and
ultraluminous infrared galaxies (U/LIRGs) (58 systems, 75 galaxies), on the
basis of integral field spectroscopy (IFS)-based high S/N integrated spectra at
galactic and sub-galactic, i.e. star forming (SF) clumps, scales.
Ambient ionized gas. The velocity dispersion of the ionized ISM in U/LIRGs
( ~ 70 kms-1) is larger than in lower luminosity local star forming
galaxies ( ~ 25 kms-1). While for isolated disc LIRGs star formation
appears to sustain turbulence, gravitational energy release associated to
interactions and mergers plays an important role driving sigma in the U/LIRG
range. We also find that the impact of an AGN in ULIRGs is strong, increasing
sigma by a factor 1.5 on average. The observed weak dependency of sigma on SFR
surface density for local U/LIRGs is in very good agreement with that measured
in some high-z sources.
Ionized outflows. The presence of ionized gas outflows in U/LIRGs seems
universal based on the detection of a broad, usually blueshifted, Halpha line.
AGNs in U/LIRGs are able to generate faster (x2) and more massive (x1.4)
ionized gas outflows than pure starbursts. The derived ionized mass loading
factors are in general below one, with only a few AGNs above this limit. Only a
small fraction of the ionized material from low mass LIRGs (log(Mdyn/Msun) <
10.4) could reach the intergalactic medium, with more massive galaxies
retaining the gas. The observed average outflow properties in U/LIRGs are
similar to high-z galaxies of comparable SFR. In the bright SF clumps found in
LIRGs, ionized gas outflows appear to be very common. For a given SFR surface
density, outflows in LIRG clumps would be about one to two orders of magnitude
less energetic than those launched by clumps in high-z SF galaxies.
We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multiwavelength follow-up spans ∼900 d, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad ∼2000 km s−1 hydrogen, He i, and O i emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 × 1050 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.
We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the All-Sky Automated Survey for Supernova survey at an absolute magnitude of MV ∼ −20.2 mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He i in early spectra, with He ii making an appearance after ∼70–100 d. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical active galactic nucleus in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photoionization. The flat Balmer decrement together with the complex profiles suggests that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He ii may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.
We present observations of ASASSN-19dj, a nearby tidal disruption event (TDE) discovered in the post-starburst galaxy KUG 0810+227 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d ≃ 98 Mpc. We observed ASASSN-19dj from −21 to 392 days relative to peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From the ASAS-SN g-band data, we determine that the TDE began to brighten on 2019 February 6.8 and for the first 25 days the rise was consistent with a flux ∝t2 power-law. ASASSN-19dj peaked in the UV/optical on 2019 March 6.5 (MJD = 58548.5) at a bolometric luminosity of L = (6.2 ± 0.2) × 1044ergs−1. Initially remaining roughly constant in X-rays and slowly fading in the UV/optical, the X-ray flux increased by over an order of magnitude ∼225 days after peak, resulting from the expansion of the X-ray emitting region. The late-time X-ray emission is well-fit by a blackbody with an effective radius of ∼1 × 1012cm and a temperature of ∼6 × 105K. The X-ray hardness ratio becomes softer after brightening and then returns to a harder state as the X-rays fade. Analysis of Catalina Real-Time Transient Survey images reveals a nuclear outburst roughly 14.5 years earlier with a smooth decline and a luminosity of LV ≥ 1.4 × 1043 erg s−1, although the nature of the flare is unknown. ASASSN-19dj occurred in the most extreme post-starburst galaxy yet to host a TDE, with Lick HδA = 7.67 ± 0.17 Å.
We present new medium resolution, optical long-slit spectra of a sample of six ultraviolet (UV)/optical and 17 X-ray-selected tidal disruption event candidate host galaxies. We measure emission line ratios from the optical spectra, finding that the large majority of hosts are quiescent galaxies, while those displaying emission lines are generally consistent with star formation dominated environments; only three sources show clear evidence of nuclear activity. We measure bulge velocity dispersions using absorption lines and infer host black hole (BH) masses using the M – σ relation. While the optical and X-ray host BH masses are statistically consistent with coming from the same parent distribution, the optical host distribution has a visible peak near $M_{\rm BH} \sim 10^6 \, \mathrm{M}_\odot$, whereas the X-ray host distribution appears flat in MBH. We find a subset of X-ray-selected candidates that are hosted in galaxies significantly less luminous (Mg ∼ −16) and less massive (stellar mass ∼ 108.5–9 M⊙) than those of optical events. Using statistical tests we find suggestive evidence that, in terms of BH mass, stellar mass, and absolute magnitude, the hard X-ray hosts differ from the UV/optical and soft X-ray samples. Similar to individual studies, we find that the size of the emission region for the soft X-ray sample is much smaller than the optical emission region, consistent with a compact accretion disc. We find that the typical Eddington ratio of the soft X-ray emission is ∼ 0.01, as opposed to the optical events which have LBB ∼ LEdd. The latter seems artificial if the radiation is produced by self-intersection shocks, and instead suggests a connection to the supermassive black hole.
We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z = 0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; the spectrum shows a persistent blackbody temperature T ≳ 20 000 K with broad H i and He ii emission; and it peaks at a blackbody luminosity of L ≈ 1044 erg s−1. The lines are initially centred at zero velocity, but by 100 d, the H i lines disappear while the He ii develops a blueshift of ≳ 5000 km s−1. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of ∼106.3 M⊙. The host is another Balmer-strong absorption galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 d are among the deepest for a TDE at this phase.
We explore the accretion states of tidal disruption events (TDEs) using a sample of seven X-ray bright sources. To this end, we estimate the relative contribution of the disc and corona to the observed X-ray emission through spectral modelling, and assess the X-ray brightness (through αox, L$_{2\ \rm keV}$, and fEdd,X) as a function of the Eddington ratio. We report strong positive correlations between αox and fEdd,bol; fEdd,X and fEdd,UV; and an anticorrelation for L$_{2\ \rm keV}$ and fEdd,UV. TDEs at high fEdd,bol have thermal dominated X-ray spectra and high (soft) αox, whereas those at low fEdd,bol show a significant power-law contribution and low (hard) αox. Similar to X-ray binaries and active galactic nuclei, the transition between X-ray spectral states occurs around fEdd,bol ≈ 0.03, although the uncertainty is large due to the small sample size. Our results suggest that X-ray surveys are more likely to discover TDEs at low fEdd,bol, whereas optical surveys are more sensitive to TDEs at high Eddington ratios. The X-ray and optical selected TDEs have different UV and X-ray properties, which should be taken into account when deriving rates, luminosity, and black hole mass functions.
We present the results from Nordic Optical Telescope and X-shooter follow-up campaigns of the tidal disruption event (TDE) iPTF16fnl, covering the first ∼100 d after the transient discovery. We followed the source photometrically until the TDE emission was no longer detected above the host galaxy light. The bolometric luminosity evolution of the TDE is consistent with an exponential decay with e-folding constant t0 = 17.6 ± 0.2 d. The early-time spectra of the transient are dominated by broad He ii λ4686, H $\beta$, H $\alpha$, and N iii λ4100 emission lines. The latter is known to be produced together with the N iii λ4640 in the Bowen fluorescence mechanism. Due to the medium-resolution X-shooter spectra we have been able to separate the Bowen blend contribution from the broad He ii emission line. The detection of the Bowen fluorescence lines in iPTF16fnl place this transient among the N-rich TDE subset. In the late-time X-shooter spectra, narrow emission lines of [O iii] and [N ii] originating from the host galaxy are detected, suggesting that the host galaxy harbours a weak active galactic nucleus in its core. The properties of all broad emission lines evolve with time. The equivalent widths follow an exponential decay compatible with the bolometric luminosity evolution. The full width at half-maximum of the broad lines decline with time and the line profiles develop a narrow core at later epochs. Overall, the optical emission of iPTF16fnl can be explained by being produced in an optically thick region in which high densities favour the Bowen fluorescence mechanism and where multiple electron scatterings are responsible for the line broadening.
A star coming too close to a supermassive black hole gets disrupted by the tidal force of the compact object in a tidal disruption event, or TDE. Following this encounter, the debris evolves into an elongated stream, half of which coming back to pericentre. Relativistic apsidal precession then leads to a self-crossing shock that initiates the formation of an accretion disc. We perform the first simulation of this process considering a parabolic encounter with a supermassive black hole, which has so far eluded investigations for computational reasons. This numerical issue is alleviated by using as initial conditions the outflow launched by the self-crossing shock according the local simulation of Lu & Bonnerot (2020). We find that the gas leaving the intersection point experiences numerous secondary shocks that result in the rapid formation of a thick and marginally bound disc. The mass distribution features two overdensities identified as spiral shocks that drive slow gas inflow along the mid-plane. Inward motion primarily takes place along the funnels of the newly formed torus, from which a fraction of the matter can get accreted. Further out, the gas moves outward forming an extended envelope completely surrounding the accretion flow. Secondary shocks heat the debris at a rate of a few times 10⁴⁴ erg s⁻¹ with a large fraction likely participating to the bolometric luminosity. These results pave the way towards a complete understanding of the early radiation from TDEs that progressively becomes accessible from observations.
Stars that pass too close to a super-massive black hole may be disrupted by strong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) which rapidly brightened and peaked in the optical/UV bands in early 2016 and subsequently decayed over the rest of the year. OGLE16aaa was detected in an XMM-Newton X-ray observation on June 9, 2016 with a flux slightly below the Swift /XRT upper limits obtained during the optical light curve peak. Between June 16–21, 2016, Swift /XRT also detected OGLE16aaa and based on the stacked spectrum, we could infer that the X-ray luminosity had jumped up by more than a factor of ten in just one week. No brightening signal was seen in the simultaneous optical/UV data to cause the X-ray luminosity to exceed the optical/UV one. A further XMM-Newton observation on November 30, 2016 showed that almost a year after the optical/UV peak, the X-ray emission was still at an elevated level, while the optical/UV flux decay had already leveled off to values comparable to those of the host galaxy. In all X-ray observations, the spectra were nicely modeled with a 50–70 eV thermal component with no intrinsic absorption, with a weak X-ray tail seen only in the November 30 XMM-Newton observation. The late-time X-ray behavior of OGLE16aaa strongly resembles the tidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint the time delay between the initial optical TDE onset and the X-ray brightening to 182 ± 5 days, which may possibly represent the timescale between the initial circularization of the disrupted star around the super-massive black hole and the subsequent delayed accretion. Alternatively, the delayed X-ray brightening could be related to a rapid clearing of a thick envelope that covers the central X-ray engine during the first six months.
We present multiwavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory survey at redshift z = 0.07897. The optical and ultraviolet (UV) light curves of the transient show a slow decay over 5 months, in agreement with previous optically discovered TDEs. It also has a comparable blackbody peak luminosity of . The inferred temperature from the optical and UV data shows a value of (3-5) ×10 ⁴ K. The transient is not detected in X-rays up to within the first 5 months after discovery. The optical spectra exhibit two distinct broad emission lines in the He ii region, and at later times also Hα emission. Additionally, emission from [N iii] and [O iii] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N v, C iv, Si iv, and possibly P v. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities cm ⁻² . This optically thick gas would also explain the nondetection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral UV line monitoring of future TDEs could test this proposal. © 2019. The American Astronomical Society. All rights reserved.
The origin of nebular He » II emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the integrated X-ray luminosity per unit of star formation rate (L X /SFR) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed He » II λ4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He ⁺ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed I(4686)/I(Hβ) intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular He » II emission in low-metallicity star-forming galaxies.
We present late-time optical spectroscopy and X-ray, UV, and optical photometry of the nearby ($d=214$ Mpc, $z=0.0479$) tidal disruption event (TDE) ASASSN-15oi. The optical spectra span 450 days after discovery and show little remaining transient emission or evolution after roughly 3 months. In contrast, the Swift and XMM-Newton observations indicate the presence of evolving X-ray emission and lingering thermal UV emission that is still present 600 days after discovery. The thermal component of the X-ray emission shows a unique, slow brightening by roughly an order of magnitude to become the dominant source of emission from the TDE at later times, while the hard component of the X-ray emission remains weak and relatively constant throughout the flare. The TDE radiated $(1.32\pm0.06)\times10^{51}$ ergs across all wavelengths, and the UV and optical emission is consistent with a power law decline and potentially indicative of a late-time shift in the power-law index that could be caused by a transition in the dominant emission mechanism.
In the past few years wide-field optical and UV transient surveys as well as X-ray telescopes have allowed us to identify a few dozen candidate tidal disruption events (TDEs). While in theory the physical processes in TDEs are expected to be ubiquitous, a few distinct classes of TDEs have been observed. Some TDEs radiate mainly in NUV/optical while others produce prominent X-rays. Moreover, relativistic jets have been observed in only a handful of TDEs. This diversity might be related to the details of the super-Eddington accretion and emission physics relevant to TDE disks. In this Letter, we utilize novel three-dimensional general relativistic radiation magnetohydrodynamics simulations to study the super-Eddington compact disk phase expected in TDEs. Consistent with previous studies, geometrically thick disks, wide-angle optically-thick fast outflows and relativistic jets are produced. The outflow density and velocity depend sensitively on the inclination angle, and hence so does the reprocessing of emission produced from the inner disk. We then use Monte-Carlo radiative transfer to calculate the reprocessed spectra and find that that the observed ratio of optical to X-ray fluxes increases with increasing inclination angle. This naturally leads to a unified model for different classes of TDEs in which the spectral properties of the TDE depend mainly on the viewing-angle of the observer with respect to the orientation of the disk.
We report on late time radio and X-ray observations of the tidal disruption event candidate ASASSN-14li, covering the first 1000 days of the decay phase. For the first $\sim200$ days the radio and X-ray emission fade in concert. This phase is better fit by an exponential decay at X-ray wavelengths, while the radio emission is well described by either an exponential or the canonical $t^{-5/3}$ decay assumed for tidal disruption events. The correlation between radio and X-ray emission during this period can be fit as $L_{R}\propto L_{X}^{1.9\pm0.2}$. After 400 days the radio emission at $15.5\,\textrm{GHz}$ has reached a plateau level of $244\pm8\,\mu\textrm{Jy}$ which it maintains for at least the next 600 days, while the X-ray emission continues to fade exponentially. This steady level of radio emission is likely due to relic radio lobes from the weak AGN-like activity implied by historical radio observations. We note that while most existing models are based upon the evolution of ejecta which are decoupled from the central black hole, the radio : X-ray correlation during the declining phase is also consistent with core jet emission coupled to a radiatively efficient accretion flow.
We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of $\sim10$ one year after its discovery, while it faded in the UV/optical by a factor of $\sim 100$. The XMM-Newton observations measure a soft X-ray blackbody component with $kT_{\rm bb} \sim 45$ eV, corresponding to radiation from several gravitational radii of a central $\sim 10^6 M_\odot$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk, and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.