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![Two models of the astrophysical neutrino flux (black lines) observed by IceCube and the corresponding cascaded gamma-ray flux (red lines) observed by Fermi. The models assume that the decay products of neutral and charged pions from pp interactions are responsible for the non-thermal emission in the Universe [67]. The thin lines represent an attempt to minimize the contribution of the pionic gamma-ray flux to the Fermi observations. It assumes an injected flux of E⁻² with exponential cutoff at low and high energy. The black data points are measured by the IceCube 3 year ‘High-Energy Starting Event’ (‘HESE’) analysis [57], the gray data points are from an IceCube analysis lowering the energy threshold for events starting in the detector even further [65].](publication/283325052/figure/fig12/AS:11431281207390492@1701182477967/Two-models-of-the-astrophysical-neutrino-flux-black-lines-observed-by-IceCube-and-the.jpg)
Two models of the astrophysical neutrino flux (black lines) observed by IceCube and the corresponding cascaded gamma-ray flux (red lines) observed by Fermi. The models assume that the decay products of neutral and charged pions from pp interactions are responsible for the non-thermal emission in the Universe [67]. The thin lines represent an attempt to minimize the contribution of the pionic gamma-ray flux to the Fermi observations. It assumes an injected flux of E⁻² with exponential cutoff at low and high energy. The black data points are measured by the IceCube 3 year ‘High-Energy Starting Event’ (‘HESE’) analysis [57], the gray data points are from an IceCube analysis lowering the energy threshold for events starting in the detector even further [65].
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![The all-particle spectrum of cosmic rays (from [4]). At the energies of...](publication/283325052/figure/fig1/AS:11431281207395008@1701182476160/The-all-particle-spectrum-of-cosmic-rays-from-4-At-the-energies-of-interest-here_Q320.jpg)
We appraise the status of high-energy neutrino astronomy and summarize the observations that define the 'IceCube puzzle.' The observations are closing in on the source candidates that may contribute to the observation. We highlight the potential of multi-messenger analysis to assist in the identification of the sources. We also give a brief overvie...
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... Charged pions decay into neutrinos: π + → µ + ν µ , followed by µ + → e +ν µ ν e , while π 0 → γγ (see Fig. 2). Since a π 0 decays into two photons for every charged pion producing a ν µνµ pair, it gives a powerful relation between the γ-ray and neutrino fluxes [13,14]: ...
The burgeoning field of multi-messenger astronomy is poised to revolutionize our understanding of the most enigmatic astrophysical phenomena in the Universe. At the same time, it has opened a new window of opportunity to probe various particle physics phenomena. This is illustrated here with a few example new physics scenarios, namely, decaying heavy dark matter, pseudo-Dirac neutrinos and light dark sector physics, for which new constraints are derived using recent multi-messenger observations.
... Although several potential candidates have been suggested as possible neutrino sources (for a review see Becker 2008;Ahlers & Halzen 2015), there is currently no clear dominant population responsible for the diffuse neutrino flux. Currently the flaring blazar TXS 0506+056 (IceCube Collaboration et al. 2018), and the nearby Seyfert galaxy NGC1068 (Collaboration et al. 2022) stand out as significant point sources in the VHE neutrino sky, but individually these sources do not contribute a significant fraction of the diffuse flux. ...
The IceCube neutrino observatory detects the diffuse astrophysical neutrino background with high significance, but the contribution of different classes of sources is not established. Because of their non-thermal spectrum, gamma-ray bursts (GRBs) are prime particle acceleration sites and one of the candidate classes for significant neutrino production. Exhaustive searches, based on stacking analysis of GRBs however could not establish the link between neutrinos and GRBs. Gamma-ray burst GRB 221009A had the highest time integrated gamma-ray flux of any detected GRB so far. The total fluence exceeds the sum of all Fermi Gamma-ray Burst Monitor (GBM) detected GRBs by a factor of two. Because it happened relatively nearby, it is one of the most favorable events for neutrino production from GRBs yet no neutrinos were detected. We calculate neutrino fluxes for this GRB in the TeV-PeV range using the most accurate, time-resolved spectral data covering the brightest intervals. We place limits on the physical parameters (Lorentz factor, baryon loading or emission radius) of the burst that are better by a factor of 2 compared to previous limits. The neutrino non-detection indicates a bulk Lorentz factor greater than 500 and possibly even 1000, consistent with other observations.
... Based on this simplified flow diagram, we expect equal fluxes of gamma rays and muon neutrinos. The flow diagram of Fig. 1 implies a multimessenger interface between the pionic gamma-ray and three-flavor neutrino flux [22]: ...
Over the past decade, neutrino astronomy has emerged as a new window into the extreme and hidden universe. Current generation experiments have detected high-energy neutrinos of astrophysical origin and identified the first sources, opening the field to discovery. Looking ahead, the authors of this Perspective identify seven major open questions in neutrino astrophysics and particle physics that could lead to transformative discoveries over the next 20 years. These multi-disciplinary questions range from understanding the vicinity of a black hole to unveiling the nature of neutrino mass, among other topics. Additionally, we critically review the current experimental capabilities and their limitations and, from there, discuss the interplay between different proposed neutrino telescope technologies and analysis techniques. The authors firmly believe that achieving the immense discovery potential over the next two decades demands a model of global partnership and complementary specialized detectors. This collaborative neutrino telescope network will pave the way for a thriving multi-messenger era, transforming our understanding of neutrino physics, astrophysics, and the extreme universe. \end{abstract}
... To date, only two HE-neutrino sources have been found (i.e, TXS 0506 +056 and NGC 1068; [5,6,31]), however several astronomical objects were suggested as possible sources (for a review see [32,33]). The particular mechanism by which the HE neutrinos are created in such environments is not yet known, however several possibilities have been investigated (e.g., [34][35][36][37][38][39][40][41]). ...
Numerous studies suggest that high-energy (HE) neutrinos and ultra-high-energy (UHE) cosmic rays could originate from extremely high-synchrotron peaked (EHSP) BL Lacs, which have been identified as effective particle accelerators. Due to the discovery of HE-neutrinos by the IceCube telescope, these hypotheses may shortly have the opportunity to be tested. In this work, we use a two-zone leptohadronic model to explain the spatial coincidence of three EHSP BL Lac: 1RXS J09462.5+010459, 1ES 1101-232, and 3HSP J095507.9+355101 with the arrival of track-like neutrinos. Our results for 1RXS J09462.5+010459 and 1ES 1101-232 indicate that the model accurately describes the electromagnetic emission and neutrino events without increasing the fluxes in the measured bands. In addition, the X-ray flaring state of 3HSP J095507.9+355101 can be explained by our model, but the measured ultraviolet flux during the neutrino arrival time window cannot be explained. For all cases, the broadband emission and neutrino arrival are better described by hard proton distributions ≈1.5. Finally, the proton luminosity required to explain the neutrino fluxes is slightly higher than the Eddington limit with a photopion efficiency of ≈0.1 for non-flaring state cases. On the other hand, for the flaring state of 3HSP J095507.9+355101, the proton luminosity must be higher than the Eddington limit at least by one order of magnitude, even if the photopion efficiency reaches unity.
... In fact, the majority of γ-NLS1s are LSPs (Abdollahi et al. 2022). In addition to the leptonic scenarios, hadronic scenarios could also play an important role in γ-NLS1s, and hence they are possible neutrino emitters (Ahlers & Halzen 2015). Recently, a cospatial incoming IceCube neutrino event is temporally coincident with a minor γ-ray flare of 1H 0323+342 (Franckowiak et al. 2020). ...
We report on the identification of a new γ -ray-emitting narrow-line Seyfert 1 galaxy ( γ -NLS1), SDSS J095909.51+460014.3 ( z = 0.399), by establishing an association with a γ -ray source 4FGL 0959.6+4606, although its low-energy counterpart was suggested to be a radio galaxy (RG) 2MASX J09591976+4603515. Wide-field Infrared Survey Explorer long-term light curves of these two sources reveal diverse infrared variability patterns. Brightenings of 2.5 mag are detected for the former source, while flux decays of 0.5 mag are found for the other one. More importantly, the time that the infrared flux of the NLS1 rises is coincident with the time of flux increase of 4FGL 0959.6+4606. At the same time, no infrared activity of the RG has been observed. A specific analysis of 15-month data from Large Area Telescope aboard Fermi , aimed at the high γ -ray flux state, yields a significant source (TS = 43). The corresponding γ -ray localisation analysis suggests that only the NLS1 falls into the uncertainty area, further supporting the updated association relationship. A broadband spectral energy distribution of SDSS J095909.51+460014.3 has been drawn and well described by the classic single-zone homogeneous leptonic jet model. Its jet properties were investigated and found to be comparable with the known γ -NLS1s.
... C 0 with C 0 = 3 × 10 −18 GeV −1 s −1 cm −2 sr −1 [24]. Although many astrophysical objects have been proposed as potential sources [for a review see 29,30], the only HE-neutrino source identified up to date is the BL Lac TXS 0506 +056 [31,32]. However, the mechanism of how the HE neutrinos are produced in that blazar is not precise, and different mechanisms have been explored [e.g., [33][34][35][36][37][38][39][40][41]. ...
Blazars have been pointed out as promising high-energy (HE) neutrinos sources, although the mechanism is still under debate. The blazars with a hard-TeV spectrum, which leptonic models can hardly explain, can be successfully interpreted in the hadronic scenarios. Recently, Aguilar et al. proposed a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known extreme BL Lacs and showed that the hadronic component could mainly interpret very-high-energy (VHE) emission. In this work, we apply this hadronic model to describe the VHE gamma-ray fluxes of 14 extreme BL Lacs and estimate the respective HE neutrino flux from charge-pion decay products. Finally, we compare our result with the diffuse flux observed by the IceCube telescope, showing that the neutrino fluxes from these objects are negligible.
... The population of HE neutrinos is distributed uniformly in the sky [1], leading to the connection between neutrinos and extragalactic sources. According to theoretical studies (e.g., [3,29]), among the most probable candidates as astrophysical counterparts of HE neutrinos, there is the class of blazars, radio-loud active galactic nuclei (AGN) with the jet being aligned with the observer's line of sight. It is expected that the very energetic processes occurring in blazar jets can accelerate cosmic rays up to the energies required for HE neutrino production [26]. ...
... C 0 with C 0 = 3 × 10 −18 GeV −1 s −1 cm −2 sr −1 [24]. Although many astrophysical objects have been proposed as potential sources [for a review see 29,30], the only HE-neutrino source identified up to date is the BL Lac TXS 0506 +056 [31,32]. However, the mechanism of how the HE neutrinos are produced in that blazar is not precise, and different mechanisms have been explored [e.g., [33][34][35][36][37][38][39][40][41]. ...
... First evidence that neutrino sources must be connected to gamma absorption was already seen in the first signal of the diffuse neutrino flux (Kimura et al. 2015;Murase et al. 2016). Even here, theoretical models are in need of gamma-ray absorption in order not to overshoot the diffuse gamma-ray flux as measured by Fermi (Murase et al. 2013;Ahlers & Halzen 2015). Further coincidences of high-energy neutrinos with blazars also point to the fact that these arrive at times of low gamma-ray activity (Kun et al. 2020). ...
On 2022 September 18, an alert by the IceCube Collaboration indicated that a ∼170 TeV neutrino arrived in directional coincidence with the blazar TXS 0506+056. This event adds to two previous pieces of evidence that TXS 0506+056 is a neutrino emitter, i.e., a neutrino alert from its direction on 2017 September 22, and a 3 σ signature of a dozen neutrinos in 2014/2015. De Bruijn el al. showed that two previous neutrino emission episodes from this blazar could be due to a supermassive binary black hole (SMBBH) central engine where jet precession close to the final coalescence of the binary results in periodic emission. This model predicted a new emission episode consistent with the 2022 September 18 neutrino observation by IceCube. Here, we show that the neutrino cadence of TXS 0506+056 is consistent with an SMBBH origin. We find that the emission episodes are consistent with an SMBBH with mass ratios q ≲ 0.3 for a total black hole mass of M tot ≳ 3 · 10 ⁸ M ⊙ . For the first time, we calculate the characteristic strain of the gravitational wave emission of the binary, and show that the merger could be detectable by LISA for black hole masses <5 · 10 ⁸ M ⊙ if the mass ratios are in the range 0.1 ≲ q ≲ 0.3. We predict that there can be a neutrino flare existing in the still-to-be-analyzed IceCube data peaking some time between 2019 August and 2021 January if a precessing jet is responsible for all three detected emission episodes. The next flare is expected to peak in the period 2023 January to 2026 August. Further observation will make it possible to constrain the mass ratio as a function of the total mass of the black hole more precisely and would open the window toward the preparation of the detection of SMBBH mergers.
... First evidence that neutrino sources must be connected to gamma-absorption was already seen in the first signal of the diffuse neutrino flux. Even here, theoretical models are in need of gamma-ray absorption in order not to overshoot the diffuse gamma-ray flux as measured by Fermi (Ahlers & Halzen 2015). Further coincidences of high-energy neutrinos with blazars also point to the fact that these arrive at times of low gamma-ray activity (Kun et al. 2020). ...
On September 18, 2022, an alert by ceCube indicated that a ~170TeV neutrino arrived in directional coincidence with the blazar TXS 0506+056. This event adds to two previous ones: a neutrino alert from its direction on September 22, 2017, and a 3sigma signature of a dozen neutrinos in 2014/2015. deBruijn 2020 showed that these two previous neutrino emission episodes could be due to a supermassive binary black hole (SMBBH) where jet precession close to final coalescence results in periodic emission. This model predicted a new emission episode consistent with the September 18, 2022 neutrino observation. Here, we show that the neutrino cadence of TXS 0506+056 is consistent with a SMBBH origin with mass ratios q<0.3 for a total black hole mass of M>3e8Msun. For the first time, we calculate the characteristic strain of the gravitational wave emission of the binary, and show that the merger could be detectable by LISA for black hole masses <5e8Msun if the mass ratios are in the range 0.1<q<0.3. We predict that there can be a neutrino flare existing in the still to be analyzed IceCube data peaking some time between 08/2019 and 01/2021 if a precessing jet is responsible for all three detected emission episodes. The next flare is expected to peak in the period 01/2023 to 08/2026. Further observation will make it possible to constrain the mass ratio as a function of the black hole mass more precisely and would open the window toward the preparation of the detection of SMBBH mergers.
