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Imprint of “Local Opacity” Effect in Gamma-Ray Spectrum of Blazar Jet
Abstract
Relativistic jets from accreting supermassive black holes at cosmological distances can be powerful emitters of γ -rays. However, the precise mechanisms and locations responsible for the dissipation of energy within these jets, leading to observable γ -ray radiation, remain elusive. We detect evidence for an intrinsic absorption feature in the γ -ray spectrum at energies exceeding 10 GeV, presumably due to the photon–photon pair production of γ -rays with low-ionization lines at the outer edge of broad-line region (BLR), during the high-flux state of the flat-spectrum radio quasar PKS 1424−418. The feature can be discriminated from the turnover at higher energies resulting from γ -ray absorption in the extragalactic background light. It is absent in the low-flux states, supporting the interpretation that powerful dissipation events within or at the edge of the BLR evolve into fainter γ -ray emitting zones outside the BLR, possibly associated with the moving very long baseline interferometry radio knots. The inferred location of the γ -ray emission zone is consistent with the observed variability timescale of the brightest flare, provided that the flare is attributed to external Compton scattering with BLR photons.
... Shorter time delays during active states imply that -ray dissipation occurs closer to the central engine, whereas radio dissipation occurs farther out in the jet. This has been observed as the absorption of high-energy photons with energies greater than 10 GeV during high states under the influence of BLR photons at sub-parsec scale jet (Agarwal et al. 2024). ...
Variable -ray flares upto minute timescales reflect extreme particle acceleration sites. However, for high-redshift blazars, the detection of such rapid variations remains limited by current telescope sensitivities. Gravitationally lensed blazars serve as powerful tools to probe -ray production zones in distant sources, with time delays between lensed signals providing crucial insights into the spatial distribution of emission regions relative to the lens's mass-weighted center. We have utilized 15 years of Fermi-LAT -ray data from direction of PKS 1830211 to understand the origin of flaring high-energy production zone at varying flux states. To efficiently estimate the (lensed) time delay, we used a machine learning-based tool - the Gaussian Process regression algorithm, in addition to - Autocorrelation function and Double power spectrum. We found a consistent time delay across all flaring activity states, indicating a similar location for the -ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 days for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the -ray emission zone is closer to the central engine, in contrast to the radio emission zone, which is expected to be much farther away. A linear relationship between lag and magnification has been observed in the identified source and echo flares. Our results suggest that the -ray emission zone originates from similar regions away from the site of radio dissipation.
... If the γ -ray emission region resides at the base of the jet, the soft photons should be dominated by the accretion disc and the hot corona (Dermer & Schlickeiser 1993;Xue et al. 2021). When the γ -ray emission region is positioned at sub-pc from the central supermassive black hole (SMBH), the soft photons predominantly originate from the broad line region (BLR; R BLR ≈ 0.1 pc; Sikora, Begelman & Rees 1994;Kaspi et al. 2007;Bentz et al. 2009;Nalewajko, Begelman, & Sikora 2014;Agarwal et al. 2024). On the other hand, if the dissipation of the γ -ray emission occurs at about 1-10 pc, the dominant soft photon source becomes the dusty torus (DT; R DT ≈ 2.5 pc; Sikora, Moderski & Madejski (2008); Zhang et al. 2024). ...
The discovery that blazars dominate the extra-galactic γ -ray sky is a triumph in the Fermi era. However, the exact location of γ -ray emission region still remains in debate. Low-synchrotron-peaked blazars (LSPs) are estimated to produce high-energy radiation through the external Compton process, thus their emission regions are closely related to the external photon fields. We employed the seed factor approach proposed by Georganopoulos et al. It directly matches the observed seed factor of each LSP with the characteristic seed factors of external photon fields to locate the γ -ray emission region. A sample of 1 138 LSPs with peak frequencies and peak luminosities was adopted to plot a histogram distribution of observed seed factors. We also collected some spectral energy distributions (SEDs) of historical flare states to investigate the variation of γ -ray emission region. Those SEDs were fitted by both quadratic and cubic functions using the Markov-chain Monte Carlo method. Furthermore, we derived some physical parameters of blazars and compared them with the constraint of internal γγ-absorption. We find that dusty torus dominates the soft photon fields of LSPs and most γ -ray emission regions of LSPs are located at 1–10 pc. The soft photon fields could also transition from dusty torus to broad line region and cosmic microwave background in different flare states. Our results suggest that the cubic function is better than the quadratic function to fit the SEDs.
Variable γ-ray flares upto minute timescales reflect extreme particle acceleration sites. However, for high-redshift blazars, the detection of such rapid variations remains limited by current telescope sensitivities. Gravitationally lensed blazars serve as powerful tools to probe γ-ray production zones in distant sources, with time delays between lensed signals providing crucial insights into the spatial distribution of emission regions relative to the lens’s mass-weighted center. We have utilized 15 years of Fermi-LAT γ-ray data from direction of PKS 1830−211 to understand the origin of flaring high-energy production zone at varying flux states. To efficiently estimate the (lensed) time delay, we used a machine learning-based tool - the Gaussian Process regression algorithm, in addition to - Autocorrelation function and Double power spectrum. We found a consistent time delay across all flaring activity states, indicating a similar location for the γ-ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 days for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the γ-ray emission zone is closer to the central engine, in contrast to the radio emission zone, which is expected to be much farther away. A linear relationship between lag and magnification has been observed in the identified source and echo flares. Our results suggest that the γ-ray emission zone originates from similar regions away from the site of radio dissipation.
We present a long-term strong correlation between millimeter (mm) radio and gamma -ray emission in the flat-spectrum radio quasar (FSRQ) PKS\,1424418. The mm --gamma -ray connection in blazars is generally thought to originate from the relativistic jet close to the central engine. We confirm a unique long-lasting mm --gamma -ray correlation of PKS\,1424418 by using detailed correlation analyses and statistical tests, and we find its physical meaning in the source. We employed sim 8.5\,yr of (sub) mm and gamma -ray light curves observed by ALMA and Fermi -LAT, respectively. From linear and cross-correlation analyses between the light curves, we found a significant, strong mm --gamma -ray correlation over the whole period. We did not find any notable time delay within the uncertainties for the mm --gamma -ray correlation, which means zero lag. The mm wave spectral index values (S>--$418 and we discuss the origin of the gamma -rays and opacity of the inner (sub)parsec-scale jet regions.
The γ -ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, H γ line, and γ -ray emissions from the FSRQ PKS 1222+216, using long-term (∼2011–2018) optical spectroscopic data from Steward Observatory and γ -ray observations from Fermi Large Area Telescope (LAT). We measured the continuum ( F C,opt ) and H γ ( F H γ ) fluxes by performing a systematic analysis of the 6029–6452 Å optical spectra. We observed stronger variability in F C,opt than F H γ , an inverse correlation between the H γ equivalent width and F C,opt , and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF ∼ 0.5) between the F γ ‐ray>100 MeV and F C,opt (6024–6092 Å) light curves with a time lag consistent with zero at the 2 σ level. We found no correlation between the F γ ‐ray>100 MeV and F H γ light curves, probably dismissing the disk contribution to the optical and γ -ray variability. The observed strong variability in the Fermi-LAT flux and F γ ‐ray>100 MeV − F C,opt correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to- γ -ray spectral energy distributions during the γ -ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the γ -ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.
Spinning black holes in the centres of galaxies can release powerful magnetised jets. When the jets are observed at angles of less than a few degrees to the line-of-sight, they are called blazars, showing variable non-thermal emission across the electromagnetic spectrum from radio waves to gamma rays. It is commonly believed that shock waves are responsible for this dissipation of jet energy. Here we show that gamma-ray observations of the blazar 3C 279 with the space-borne telescope Fermi-LAT reveal a characteristic peak-in-peak variability pattern on time scales of minutes expected if the particle acceleration is instead due to relativistic magnetic reconnection. The absence of gamma-ray pair attenuation shows that particle acceleration takes place at a distance of ten thousand gravitational radii from the black hole where the fluid dynamical kink instability drives plasma turbulence. Blazars show variable non-thermal emission across the electromagnetic spectrum from radio waves to gamma rays. Here, the authors show blazar 3C 279 reveals a characteristic peak-in-peak variability pattern on time scales of minutes if particle acceleration is due to relativistic magnetic reconnection.
The last few years have seen gamma-ray astronomy maturing and advancing in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter, depending on the source. This review focuses on some of the key science issues and conceptual developments concerning the timing characteristics of active galactic nuclei (AGN) at gamma-ray energies. It highlights the relevance of adequate statistical tools and illustrates that the developments in the gamma-ray domain bear the potential to fundamentally deepen our understanding of the nature of the emitting source and the link between accretion dynamics, black hole physics, and jet ejection.
Outbursts from gamma-ray quasars provide insights on the relativistic jets of
active galactic nuclei and constraints on the diffuse radiation fields that
fill the Universe. The detection of significant emission above 100 GeV from a
distant quasar would show that some of the radiated gamma rays escape
pair-production interactions with low-energy photons, be it the extragalactic
background light (EBL), or the radiation near the supermassive black hole lying
at the jet's base. VERITAS detected gamma-ray emission up to 200 GeV from PKS
1441+25 (z=0.939) during April 2015, a period of high activity across all
wavelengths. This observation of PKS 1441+25 suggests that the emission region
is located thousands of Schwarzschild radii away from the black hole. The
gamma-ray detection also sets a stringent upper limit on the near-ultraviolet
to near-infrared EBL intensity, suggesting that galaxy surveys have resolved
most, if not all, of the sources of the EBL at these wavelengths.
Context. Blazars are a subset of active galactic nuclei (AGN) with jets that
are oriented along our line of sight. Variability and spectral energy
distribution (SED) studies are crucial tools for understanding the physical
processes responsible for observed AGN emission.
Aims. We report peculiar behaviour in the bright gamma-ray blazar PKS
1424-418 and use its strong variability to reveal information about the
particle acceleration and interactions in the jet. Methods. Correlation
analysis of the extensive optical coverage by the ATOM telescope and nearly
continuous gamma-ray coverage by the Fermi Large Area Telescope is combined
with broadband, time-dependent modeling of the SED incorporating supplemental
information from radio and X-ray observations of this blazar.
Results. We analyse in detail four bright phases at optical-GeV energies.
These flares of PKS 1424-418 show high correlation between these energy ranges,
with the exception of one large optical flare that coincides with relatively
low gamma-ray activity. Although the optical/gamma-ray behaviour of PKS
1424-418 shows variety, the multiwavelength modeling indicates that these
differences can largely be explained by changes in the flux and energy spectrum
of the electrons in the jet that are radiating. We find that for all flares the
SED is adequately represented by a leptonic model that includes inverse Compton
emission from external radiation fields with similar parameters.
Conclusions. Detailed studies of individual blazars like PKS 1424-418 during
periods of enhanced activity in different wavebands are helping us identify
underlying patterns in the physical parameters in this class of AGN.
We extend our previous model-independent determination of the intergalactic
background light (IBL), based purely on galaxy survey data, out to a wavelength
of 5 microns. Our approach enables us to constrain the range of photon
densities, based on the uncertainties from observationally determined
luminosity densities and colors. We further determine a 68% confidence upper
and lower limit on the opacity of the universe to gamma-rays up to energies of
1.6/(1+z) TeV. A comparison of our lower limit redshift-dependent opacity
curves to the opacity limits derived from the results of both ground-based air
Cherenkov telescope and Fermi-LAT observations of PKS 1424+240 allows us to
place a new upper limit on the redshift of this source, independent of IBL
modeling.
The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view (FoV), high-energy γ-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its preflight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 × 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 (x, y) tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an eight-layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the high-energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large FoV (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (1) permit rapid notification of high-energy γ-ray bursts and transients and facilitate monitoring of variable sources, (2) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (3) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (4) localize point sources to 0.3-2 arcmin, (5) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (6) measure the diffuse isotropic γ-ray background up to TeV energies, and (7) explore the discovery space for dark matter.
The quasar PKS 1510-089 (z=0.361) was observed with the H.E.S.S. array of
imaging atmospheric Cherenkov telescopes during high states in the optical and
GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV)
emission. VHE \gamma-rays were detected with a statistical significance of 9.2
standard deviations in 15.8 hours of H.E.S.S. data taken during March and April
2009. A VHE integral flux of I(0.15 TeV < E < 1.0 TeV) = (1.0 +- 0.2 (stat) +-
0.2 (sys) x 10^{-11} cm^{-2}s^{-1} is measured. The best-fit power law to the
VHE data has a photon index of \Gamma=5.4 +- 0.7 (stat) +- 0.3 (sys). The GeV
and optical light curves show pronounced variability during the period of
H.E.S.S. observations. However, there is insufficient evidence to claim
statistically significant variability in the VHE data. Because of its
relatively high redshift, the VHE flux from PKS 1510-089 should suffer
considerable attenuation in the intergalactic space due to the extragalactic
background light (EBL). Hence, the measured \gamma-ray spectrum is used to
derive upper limits on the opacity due to EBL, which are found to be comparable
with the previously derived limits from relatively-nearby BL Lac objects.
Unlike typical VHE-detected blazars where the broadband spectrum is dominated
by non-thermal radiation at all wavelengths, the quasar PKS 1510-089 has a
bright thermal component in the optical to UV frequency band. Among all VHE
detected blazars, PKS 1510-089 has the most luminous broad line region (BLR).
The detection of VHE emission from this quasar indicates a low level of
\gamma-\gamma absorption on the internal optical to UV photon field.
We report the detection of a statistically significant flare-like event in the Mg II λ2800 emission line of 3C 454.3 during the outburst of autumn 2010. The highest levels of emission line flux recorded over the monitoring period (2008-2011) coincide with a superluminal jet component traversing through the radio core. This finding crucially links the broad emission line fluctuations to the non-thermal continuum emission produced by relativistically moving material in the jet and hence to the presence of broad-line region clouds surrounding the radio core. If the radio core were located at several parsecs from the central black hole, then our results would suggest the presence of broad-line region material outside the inner parsec where the canonical broad-line region is envisaged to be located. We briefly discuss the implications of broad emission line material ionized by non-thermal continuum in the context of virial black hole mass estimates and gamma-ray production mechanisms.
We present a new model of the extragalactic background light (EBL) and
corresponding gamma-gamma opacity for intergalactic gamma-ray absorption from z
= 0 up to z = 10, based on a semi-analytical model of hierarchical galaxy
formation that reproduces key observed properties of galaxies at various
redshifts. Including the potential contribution from Population III stars and
following the cosmic reionization history in a simplified way, the model is
also broadly consistent with available data concerning reionization,
particularly the Thomson scattering optical depth constraints from WMAP. In
comparison with previous EBL studies up to z ~ 3-5, our predicted gamma-gamma
opacity is in general agreement for observed gamma-ray energy below 400/(1 + z)
GeV, whereas it is a factor of ~ 2 lower above this energy because of a
correspondingly lower cosmic star formation rate, even though the observed UV
luminosity is well reproduced by virtue of our improved treatment of dust
obscuration and direct estimation of star formation rate. The horizon energy at
which the gamma-ray opacity is unity does not evolve strongly beyond z ~ 4 and
approaches ~ 20 GeV. The contribution of Population III stars is a minor
fraction of the EBL at z = 0, and is also difficult to distinguish through
gamma-ray absorption in high-z objects, even at the highest levels allowed by
the WMAP constraints. Nevertheless, the attenuation due to Population II stars
should be observable in high-z gamma-ray sources by telescopes such as Fermi or
CTA and provide a valuable probe of the evolving EBL in the rest-frame UV. The
detailed results of our model are publicly available in numerical form at the
URL http://www.slac.stanford.edu/~yinoue/Download.html.
The light emitted by stars and accreting compact objects through the history of the universe is encoded in the intensity of
the extragalactic background light (EBL). Knowledge of the EBL is important to understand the nature of star formation and
galaxy evolution, but direct measurements of the EBL are limited by galactic and other foreground emissions. Here, we report
an absorption feature seen in the combined spectra of a sample of gamma-ray blazars out to a redshift of z ∼ 1.6. This feature is caused by attenuation of gamma rays by the EBL at optical to ultraviolet frequencies and allowed us
to measure the EBL flux density in this frequency band.
The flux variability of blazars at very high energies does not have a clear
origin. Flux variations on time scales down to the minute suggest that
variability originates in the jet, where a relativistic boost can shorten the
observed time scale, while the linear relation between the flux and its RMS or
the skewness of the flux distribution suggests that the variability stems from
multiplicative processes, which are associated in some models with the
accretion disk. We study the RMS-flux relation and emphasize its link to Pareto
distributions, characterized by a power-law probability density function. Such
distributions are naturally generated within a minijets- in-a-jet statistical
model, in which boosted emitting regions are isotropically oriented within the
bulk relativistic flow of a jet. We prove that, within this model, the flux of
a single minijet is proportional to its RMS. This relation still holds when
considering a large number of emitting regions, for which the distribution of
the total flux is skewed and could be interpreted as being log-normal. The
minijets-in-a-jet statistical model reconciles the fast variations and the
statistical properties of the flux of blazars at very high energies.
In this paper, we study the photon-photon pair production optical depth for gamma rays with energies from 10 to 200 GeV emitted by powerful blazars due to the diffuse radiation field of the broad-line region (BLR). There are four key parameters in the BLR model employed to determine the γ-γ attenuation optical depth of these gamma rays. They are the gamma-ray emitting radius Rγ, the BLR luminosity LBLR, the BLR half thickness h, and the ratio τBLR/fcov of the Thomson optical depth to the covering factor of BLR. For FSRQs, on average, it is impossible for gamma rays with energies from 10 to 200 GeV to escape from the diffuse radiation field of the BLR. If GLAST could detect these gamma rays for most of FSRQs, the gamma-ray emitting region is likely to be outside the cavity formed by the BLR. Otherwise, the emitting region is likely to be inside the BLR cavity. As examples, we estimate the photon-photon absorption optical depth of gamma rays with energies from 10 to 200 GeV for two powerful blazars, HFSRQ PKS 0405-123 and FSRQ 3C 279. Comparing our results with GLAST observations in the future could test whether the model employed and the relevant assumptions in this paper are reliable and reasonable, and then limit constraints on the position of the gamma-ray emitting region relative to the BLR and the properties of the BLR.
This paper addresses the problem of detecting and characterizing local
variability in time series and other forms of sequential data. The goal is to
identify and characterize statistically significant variations, at the same
time suppressing the inevitable corrupting observational errors. We present a
simple nonparametric modeling technique and an algorithm implementing it - an
improved and generalized version of Bayesian Blocks (Scargle 1998) - that finds
the optimal segmentation of the data in the observation interval. The structure
of the algorithm allows it to be used in either a real-time trigger mode, or a
retrospective mode. Maximum likelihood or marginal posterior functions to
measure model fitness are presented for events, binned counts, and measurements
at arbitrary times with known error distributions. Problems addressed include
those connected with data gaps, variable exposure, extension to piecewise
linear and piecewise exponential representations, multi-variate time series
data, analysis of variance, data on the circle, other data modes, and dispersed
data. Simulations provide evidence that the detection efficiency for weak
signals is close to a theoretical asymptotic limit derived by (Arias-Castro,
Donoho and Huo 2003). In the spirit of Reproducible Research (Donoho et al.
2008) all of the code and data necessary to reproduce all of the figures in
this paper are included as auxiliary material.
Very high energy (VHE) γ-ray emission from the flat spectrum radio quasar (FSRQ) PKS 1222+21 (4C 21.35, z = 0.432) was detected with the MAGIC Cherenkov telescopes during a short observation (~0.5 hr) performed on 2010 June 17. The MAGIC detection coincides with high-energy MeV/GeV γ-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE spectrum measured by MAGIC extends from about 70 GeV up to at least 400 GeV and can be well described by a power-law dN/dE ∝ E
–Γ with a photon index Γ = 3.75 ± 0.27stat ± 0.2syst. The averaged integral flux above 100 GeV is (4.6 ± 0.5) × 10–10 cm–2 s–1 (~1 Crab Nebula flux). The VHE flux measured by MAGIC varies significantly within the 30 minute exposure implying a flux doubling time of about 10 minutes. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light (EBL), can be described by a single power law with photon index 2.72 ± 0.34 between 3 GeV and 400 GeV, and is consistent with emission belonging to a single component in the jet. The absence of a spectral cutoff constrains the γ-ray emission region to lie outside the broad-line region, which would otherwise absorb the VHE γ-rays. Together with the detected fast variability, this challenges present emission models from jets in FSRQs. Moreover, the combined Fermi/LAT and MAGIC spectral data yield constraints on the density of the EBL in the UV-optical to near-infrared range that are compatible with recent models.
In this paper we report on the two-year-long Fermi-LAT observation of the
peculiar blazar 4C +21.35 (PKS 1222+216). This source was in a quiescent state
from the start of science operations of the Fermi Gamma-ray Space Telescope in
2008 August until 2009 September, and then became more active, with gradually
increasing flux and some moderately-bright flares. In 2010 April and June, 4C
+21.35 underwent a very strong GeV outburst composed of several major flares
characterized by rise and decay timescales of the order of a day. During the
outburst, the GeV spectra of 4C +21.35 displayed a broken power-law form with
spectral breaks observed near 1-3 GeV photon energies. We demonstrate that, at
least during the major flares, the jet in 4C +21.35 carried a total kinetic
luminosity comparable to the total accretion power available to feed the
outflow. We also discuss the origin of the break observed in the flaring
spectra of 4C +21.35. We show that, in principle, a model involving
annihilation of the GeV photons on the He II Lyman recombination continuum and
line emission of "broad line region" clouds may account for such. However, we
also discuss the additional constraint provided by the detection of 4C +21.35
at 0.07-0.4 TeV energies by the MAGIC telescope, which coincided with one of
the GeV flares of the source. We argue that there are reasons to believe that
the lesssim,TeV emission of 4C +21.35 (as well as the GeV emission of the
source, if co-spatial), is not likely to be produced inside the broad line
region zone of highest ionization (,cm from the nucleus), but
instead originates further away from the active center, namely around the
characteristic scale of the hot dusty torus surrounding the 4C +21.35 nucleus
(,cm).
Context. 3C 279, the first quasar discovered to emit VHE gamma-rays by the
MAGIC telescope in 2006, was reobserved by MAGIC in January 2007 during a major
optical flare and from December 2008 to April 2009 following an alert from the
Fermi space telescope on an exceptionally high gamma -ray state.
Aims. The January 2007 observations resulted in a detection on January 16
with significance 5.2 sigma, corresponding to a F(> 150 GeV) (3.8 \pm 0.8)
\cdot 10^-11 ph cm^-2 s^-1 while the overall data sample does not show
significant signal. The December 2008 - April 2009 observations did not detect
the source. We study the multiwavelength behavior of the source at the epochs
of MAGIC observations, collecting quasi-simultaneous data at optical and X-ray
frequencies and for 2009 also gamma-ray data from Fermi.
Methods. We study the light curves and spectral energy distribution of the
source. The spectral energy distributions of three observing epochs (including
the February 2006, which has been previously published in Albert et al. 2008a)
are modeled with one-zone inverse Compton models and the emission on January
16, 2007 also with two zone model and with a lepto-hadronic model.
Results. We find that the VHE gamma-ray emission detected in 2006 and 2007
challenges standard one-zone model, based on relativistic electrons in a jet
scattering broad line region photons, while the other studied models fit the
observed spectral energy distribution more satisfactorily.
We report about the search for short-term variability in the high-energy
gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273,
PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2
s^-1 for at least one day. Although, the statistics was not yet sufficient to
effectively measure the characteristic time scale, it allowed us to set tight
upper limits on the observed doubling time scale (< 2-3 hours) -- the smallest
measured to date at MeV energies --, which can constrain the size of the
gamma-ray emitting region. The results obtained in the present work favor the
hypothesis that gamma rays are generated inside the broad-line region.
... A strict lower-limit flux for the evolving extragalactic background light (and in particular the cosmic infrared background) has been calculated up to redshift of 5. The computed flux is below the existing upper limits from direct observations, and in agreement with all existing limits derived from very-high energy gamma-ray observations. The corrected spectra are still in agreement with simple theoretical predictions. The derived strict lower-limit EBL flux is very close to the upper limits from gamma-ray observations. This is true for the present day EBL but also for the diffuse flux at higher redshift. If future detections of high redshift gamma-ray sources require a lower EBL flux than derived here, the physics assumptions used to derive the upper limits have to be revised. The lower-limit EBL model is not only needed for absorption features in AGN and other gamma-ray sources but is also essential when alternative particle processes are tested, which could prevent the high energy gamma-rays from being absorbed. It can also be used for a quaranteed interaction of cosmic-ray particles. The model is available online. Comment: 12 pages, 6 figures, accepted by A&A
The existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic. M87, the nearest such object, is puzzling in that although it has very massive central black hole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have a rather low infrared luminosity. Using arguments from unification theory of active galactic nuclei, we have earlier suggested that the inner parsec-scale region of M87 could still harbour a small torus sufficiently cold such that its infrared emission is dwarfed by the jet emission. The infrared emission from even a small cold torus could affect through photon-photon pair production interactions the escape of 100 GeV to TeV energy gamma rays from the central parsec of M87. The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactions of gamma-rays produced in the inner jet of M87 with the weak infrared radiation expected from a possible dusty small-scale torus, and show that the HEGRA detection shows that the temperature of any torus surrounding the gamma-ray emission region must be cooler than about 250 K. We suggest that if no gamma-rays are in future detected during extreme flaring activity in M87 at other wavelength, this may be expected because of torus heating. Comment: 7 pages, submitted to Prog. Theor. Phys. Suppl., ps file
The background radiations in the optical and the infrared constitute a
relevant cause of energy loss in the propagation of high energy particles
through space. In particular, TeV observations with Cherenkov telescopes of
extragalactic sources are influenced by the opacity effects due to the
interaction of the very high-energy source photons with the background light.
With the aim of assessing with the best possible detail these opacity terms, we
have modelled the extragalactic optical and IR backgrounds using available
information on cosmic sources in the universe from far-UV to sub-mm wavelengths
over a wide range of cosmic epochs. We have exploited the relevant cosmological
survey data - including number counts, redshift distributions, luminosity
functions - from ground-based observatories in the optical, near-IR, and
sub-mm, as well as multi-wavelength information coming from space telescopes,
HST, ISO and Spitzer. Additional constraints have been used from direct
measurements or upper limits on the extragalactic backgrounds by dedicated
missions (COBE). All data were fitted and interpolated with a multi-wavelength
backward evolutionary model, allowing us to estimate the background photon
density and its redshift evolution. From the redshift-dependent background
spectrum, the photon-photon opacities for sources of high-energy emission at
any redshifts were then computed. The same results can also be used to compute
the optical depths for any kind of processes in the intergalactic space
involving interactions with background photons (like scattering of cosmic-ray
particles). We have applied our photon-photon opacity estimates to the analysis
of spectral data at TeV energies on a few BLAZARs of particular interest.
[abridged]
Bearing on the model for the time-dependent metagalactic radiation field developed in the first paper of this series, we compute the gamma-ray attenuation due to pair production in photon-photon scattering. Emphasis is on the effects of varying the star formation rate and the fraction of UV radiation assumed to escape from the star forming regions, the latter being important mainly for high-redshift sources. Conversely, we investigate how the metagalactic radiation field can be measured from the gamma-ray pair creation cutoff as a function of redshift, the Fazio-Stecker relation. For three observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of gamma-ray attenuation on their spectra in detail.
We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit cutoff at 13.6 eV in a Lambda-CDM universe with and . Our galaxy evolution model gives results which are consistent with Spitzer deep number counts and the spectral energy distribution of the extragalactic background radiation. We use our photon density results to extend previous work on the absorption of high energy gamma-rays in intergalactic space owing to interactions with low energy photons and the 2.7 K cosmic background radiation. We calculate the optical depth of the universe, tau, for gamma-rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from ~0 to 5. We also give an analytic fit with numerical coefficients for approximating . As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the H.E.S.S. air Cherenkov gamma-ray telescope array. Comment: final version to be published in ApJ
The evolution of the spectral energy distribution during flares constrains models of particle acceleration in blazar jets. The archetypical blazar BL Lacertae provided a unique opportunity to study spectral variations during an extended strong flaring episode from 2020 to 2021. During its brightest γ-ray state, the observed flux (0.1–300 GeV) reached up to , with sub-hour-scale variability. The synchrotron hump extended into the X-ray regime showing a minute-scale flare with an associated peak shift of inverse-Compton hump in γ-rays. In shock acceleration models, a high Doppler factor value >100 is required to explain the observed rapid variability, change of state, and γ-ray peak shift. Assuming particle acceleration in minijets produced by magnetic reconnection during flares, on the other hand, alleviates the constraint on required bulk Doppler factor. In such jet-in-jet models, observed spectral shift to higher energies (towards TeV regime) and simultaneous rapid variability arises from the accidental alignment of a magnetic plasmoid with the direction of the line of sight. We infer a magnetic field of ∼0.6 G in a reconnection region located at the edge of broad-line region (∼0.02 pc). The scenario is further supported by lognormal flux distribution arising from merging of plasmoids in reconnection region.
A study of blazar PKS 1424−418 was carried out using multi-waveband data collected by the Fermi Large Area Telescope (Fermi-LAT), Swift X-Ray Telescope (Swift-XRT), Swift UltraViolet and Optical Telescope (Swift-UVOT), and Small and Moderate Aperture Research Telescope System (SMARTS) between MJD 56000 and MJD 56600 (2012 March 14–2013 November 4). Two flaring episodes were identified by analysing the gamma-ray light curve. Simultaneous multi-waveband spectral energy distributions (SEDs) were obtained for those two flaring periods. A cross-correlation analysis of infrared (IR)–optical and γ-ray data suggested that the emission originated from the same region. We have set a lower limit for the Doppler factor using the highest energy photon observed from this source during the flaring periods, which should be >12. The broad-band emission mechanism was studied by modelling the SED using the leptonic emission mechanism.
We report a characterization of the multi-band flux variability and correlations of the nearby (z=0.031) blazar Markarian 421 (Mrk 421) using data from Metsähovi, Swift, Fermi-LAT, MAGIC, FACT and other collaborations and instruments from November 2014 till June 2016. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F>1TeV < 1.7× 10−12 ph cm−2 s−1) and in the 2-10 keV (X-ray) band (F2 − 10 keV < 3.6 × 10−11 erg cm−2 s−1), during which the Swift-BAT data suggests an additional spectral component beyond the regular synchrotron emission.The highest flux variability occurs in X-rays and very-high-energy (E>0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRkeV and HRTeV show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 days centered at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 days centered at a time lag of days.This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a LogNormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process.
Almost 10 yr of γ -ray observations with the Fermi Large Area Telescope have revealed extreme γ -ray outbursts from flat spectrum radio quasars (FSRQs), temporarily making these objects the brightest γ -ray emitters in the sky. Yet, the location and mechanisms of the γ -ray emission remain elusive. We characterize long-term γ -ray variability and the brightest γ -ray flares of six FSRQs. Consecutively zooming in on the brightest flares, which we identify in an objective way through Bayesian blocks and a hill-climbing algorithm, we find variability on subhour timescales and as short as minutes for two sources in our sample (3C 279 and CTA 102) and weak evidence for variability at timescales less than the Fermi satellite’s orbit of 95 minutes for PKS 1510–089 and 3C 454.3. This suggests extremely compact emission regions in the jet. We do not find any signs of γ -ray absorption in the broad-line region (BLR), which indicates that γ -rays are produced at distances greater than hundreds of gravitational radii from the central black hole. This is further supported by a cross-correlation analysis between γ -ray and radio/millimeter light curves, which is consistent with γ -ray production at the same location as the millimeter core for 3C 273, CTA 102, and 3C 454.3. The inferred locations of the γ -ray production zones are still consistent with the observed decay times of the brightest flares if the decay is caused by external Compton scattering with BLR photons. However, the minute-scale variability is challenging to explain in such scenarios.
Context . PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either during long high states of optical and GeV activity or during short flares.
Aims . We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood.
Methods . PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi -LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models.
Results . The MAGIC telescopes collected 75 hr of data during times when the Fermi -LAT flux measured above 1 GeV was below 3 × 10 ⁻⁸ cm ⁻² s ⁻¹ , which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5 σ ) VHE gamma-ray emission at the level of (4.27 ± 0.61 stat ) × 10 ⁻¹² cm ⁻² s ⁻¹ above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR.
The gamma-ray emission in broad-line blazars is generally explained as inverse Compton (IC) radiation of relativistic electrons in the jet scattering optical-UV photons from the Broad Line Region (BLR), the so-called BLR External Compton scenario. We test this scenario on the Fermi gamma-ray spectra of 106 broad-line blazars detected with the highest significance or largest BLR, by looking for cut-off signatures at high energies compatible with gamma-gamma interactions with BLR photons. We do not find evidence for the expected BLR absorption. For 2/3 of the sources, we can exclude any significant absorption (), while for the remaining 1/3 the possible absorption is constrained to be 1.5-2 orders of magnitude lower than expected. This result holds also dividing the spectra in high and low-flux states, and for powerful blazars with large BLR. Only 1 object out of 10 seems compatible with substantial attenuation (). We conclude that for 9 out of 10 objects, the jet does not interact with BLR photons. Gamma-rays seem either produced outside the BLR most of the time, or the BLR is ~100x larger than given by reverberation mapping. This means that i) External Compton on BLR photons is disfavoured as the main gamma-ray mechanism, vs IC on IR photons from the torus or synchrotron self-Compton; ii) the Fermi gamma-ray spectrum is mostly intrinsic, determined by the interaction of the particle distribution with the seed-photons spectrum; iii) without suppression by the BLR, broad-line blazars can become copious emitters above 100 GeV, as demonstrated by 3C454.3. We expect the CTA sky to be much richer of broad-line blazars than previously thought.
The GeV-range spectra of blazars are shaped not only by non-thermal emission processes internal to the relativistic jet but also by external pair-production absorption on the thermal emission of the accretion disc and the broad-line region (BLR). For the first time, we compute here the pair-production opacities in the GeV range produced by a realistic BLR accounting for the radial stratification and radiation anisotropy. Using photoionization modelling with the CLOUDY code, we calculate a series of BLR models of different sizes, geometries, cloud densities, column densities and metallicities. The strongest emission features in the model BLR are Ly and HeII Ly. Contribution of recombination continua is smaller, especially for hydrogen, because Ly continuum is efficiently trapped inside the large optical depth BLR clouds and converted to Lyman emission lines and higher-order recombination continua. The largest effects on the gamma-ray opacity are produced by the BLR geometry and localization of the gamma-ray source. We show that when the gamma-ray source moves further from the central source, all the absorption details move to higher energies and the overall level of absorption drops because of decreasing incidence angles between the gamma-rays and BLR photons. The observed positions of the spectral breaks can be used to measure the geometry and the location of the gamma-ray emitting region relative to the BLR. Strong dependence on geometry means that the soft photons dominating the pair-production opacity may be actually produced by a different population of BLR clouds than the bulk of the observed broad line emission.
We reanalyze Fermi/LAT gamma-ray spectra of bright blazars with a higher
photon statistics than in previous works and with new Pass 7 data
representation. In the spectra of the brightest blazar 3C 454.3 and possibly of
4C +21.35 we detect breaks at 5 GeV (in the rest frame) associated with the
photon-photon pair production absorption by He II Lyman continuum (LyC). We
also detect confident breaks at 20 GeV associated with hydrogen LyC both in the
individual spectra and in the stacked redshift-corrected spectrum of several
bright blazars. The detected breaks in the stacked spectra univocally prove
that they are associated with atomic ultraviolet emission features of the
quasar broad-line region (BLR). The dominance of the absorption by hydrogen Ly
complex over He II, rather small detected optical depth, and the break energy
consistent with the head-on collisions with LyC photons imply that the
gamma-ray emission site is located within the BLR, but most of the BLR emission
comes from a flat disk-like structure producing little opacity. Alternatively,
the LyC emission region size might be larger than the BLR size measured from
reverberation mapping, and/or the gamma-ray emitting region is extended. These
solutions would resolve a long-standing issue how the multi-hundred GeV photons
can escape from the emission zone without being absorbed by softer photons.
Attenuation of high-energy gamma-rays by pair production with
ultraviolet, optical and infrared (IR) extragalactic background light
(EBL) photons provides a link between the history of galaxy formation
and high-energy astrophysics. We present results from our latest
semi-analytic models (SAMs), which employ the main ingredients thought
to be important to galaxy formation and evolution, as well as an
improved model for reprocessing of starlight by dust to mid- and far-IR
wavelengths. These SAMs are based upon a Λ cold dark matter
hierarchical structural formation scenario, and are successful in
reproducing a large variety of observational constraints such as number
counts, luminosity and mass functions and colour bimodality. Our
fiducial model is based upon a Wilkinson Microwave Anisotropy Probe
5-year cosmology, and treats dust emission using empirical templates.
This model predicts a background flux considerably lower than optical
and near-IR measurements that rely on subtraction of zodiacal and
galactic foregrounds, and near the lower bounds set by number counts of
resolvable sources at a large number of wavelengths. We also show the
results of varying cosmological parameters and dust attenuation model
used in our SAM. For each EBL prediction, we show how the optical depth
due to electron-positron pair production is affected by redshift and
gamma-ray energy, and the effect of gamma-ray absorption on the spectra
of a variety of extragalactic sources. We conclude with a discussion of
the implications of our work, comparisons to other models and key
measurements of the EBL and a discussion of how the burgeoning science
of gamma-ray astronomy will continue to help constrain cosmology. The
low EBL flux predicted by our fiducial model suggests an optimistic
future for further studies of distant gamma-ray sources.
We present multi-epoch optical observations of the blazar 3C 454.3 (z =
0.859) from 2008 August through 2011 December, using the SMARTS Consortium
1.5m+RCSpectrograph and 1.3m+ANDICAM in Cerro Tololo, Chile. The spectra reveal
that the broad optical emission lines Mg II, H-beta and H-gamma are far less
variable than the optical or gamma-ray continuum. Although, the gamma-rays
varied by a factor of 100 above the EGRET era flux, the lines generally vary by
a factor of 2 or less. Smaller variations in the gamma-ray flux did not produce
significant variation in any of the observed emission lines. Therefore, to
first order, the ionizing flux from the disk changes only slowly during large
variations of the jet. However, two exceptions in the response of the broad
emission lines are reported during the largest gamma-ray flares in 2009
December and 2010 November, when significant deviations from the mean line flux
in H-gamma and Mg II were observed. H-gamma showed a maximum 3-sigma and
4-sigma deviation in each flare, respectively, corresponding to a factor of 1.7
and 2.5 increase in flux. Mg II showed a 2-sigma deviation in both flares; no
variation was detected in H-beta during either flare. These significant
deviations from the mean line flux also coincide with 7mm core ejections
reported previously (Jorstad et al. 2012). The correlation of the increased
emission line flux with mm core ejections, and gamma-ray, optical and UV flares
suggests that the broad line region extends beyond the gamma-emitting region
during the 2009 and 2010 flares.
The broad-line region (BLR) is an important component of blazars, especially
for the flat spectrum radio quasars (FSRQs). The soft photons arising from the
BLR will substantially affect the transparency of the gamma-ray photons
produced in the relativistic jet. In the paper, we study the effect of the
geometrical structure of the BLR on the absorption of gamma-rays. We find that
the gamma-ray optical depth strongly depends on the geometrical structure of
the BLR. For a "flat" geometry of the BLR, the gamma-ray photons with specified
energies could escape transparently even their emission region locates inside
the cavity of the BLR.
Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer
excellent conditions for efficient particle acceleration in internal and
external shocks, turbulence, and magnetic reconnection events. The jets as well
as particle accelerating regions close to the supermassive black holes
(hereafter SMBH) at the intersection of plasma inflows and outflows, can
produce readily detectable very high energy gamma-ray emission. As of now, more
than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the
present ground-based gamma-ray telescopes, which represents more than one third
of the cosmic sources detected so far in the VHE gamma-ray regime. The future
Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the
VHE range by about one order of magnitude, shedding new light on AGN population
studies, and AGN classification and unification schemes. CTA will be a unique
tool to scrutinize the extreme high-energy tail of accelerated particles in
SMBH environments, to revisit the central engines and their associated
relativistic jets, and to study the particle acceleration and emission
mechanisms, particularly exploring the missing link between accretion physics,
SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an
extremely rewarding observing program which will inform us about the inner
workings and evolution of AGN. Furthermore these AGN are bright beacons of
gamma-rays which will allow us to constrain the extragalactic infrared and
optical backgrounds as well as the intergalactic magnetic field, and will
enable tests of quantum gravity and other "exotic" phenomena.
We reconstruct \gamma-ray opacity of the Universe out to z<3-4 using an
extensive library of 342 observed galaxy luminosity function surveys extending
to high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic)
providing for the first time a robust empirical calculation of the
\gamma\gamma-optical depth out to several TeV. Here, we use the same database
as Helgason et al. 2012 where the EBL was reconstructed from luminosity
functions out to 4.5mic and was shown to recover observed galaxy counts to high
accuracy. We extend our earlier library of LFs to 25mic such that it covers the
energy range of pair production with \gamma-rays 1) in the entire Fermi/LAT
energy range, and 2) at higher TeV energies probed by ground-based Cherenkov
telescopes. In the absence of significant contributions to the cosmic diffuse
background from unknown populations, such as the putative Population III era
sources, the Universe appears to be largely transparent to \gamma-rays at all
Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at
z<0.2 and reaching \tau~10 at z=1. Comparing with the currently available
Fermi/LAT GRB and blazar data shows that there is room for significant
emissions originating in the first stars era.
We show that the rms–flux relation recently discovered in the X-ray light curves of active galactic nuclei (AGN) and X-ray binaries (XRBs) implies that the light curves have a formally non-linear, exponential form, provided the rms–flux relation applies to variations on all time-scales (as it appears to). This phenomenological model implies that stationary data will have a lognormal flux distribution. We confirm this result using an observation of Cyg X-1, and further demonstrate that our model predicts the existence of the powerful millisecond flares observed in Cyg X-1 in the low/hard state, and explains the general shape and amplitude of the bicoherence spectrum in that source. Our model predicts that the most variable light curves will show the most extreme non-linearity. This result can naturally explain the apparent non-linear variability observed in some highly variable narrow line Seyfert 1 (NLS1) galaxies, as well as the low states observed on long time-scales in the NLS1 NGC 4051, as being nothing more than extreme manifestations of the same variability process that is observed in XRBs and less variable AGN. That variability process must be multiplicative (with variations coupled together on all time-scales) and cannot be additive (such as shot-noise), or related to self-organized criticality, or result from completely independent variations in many separate emitting regions. Successful models for variability must reproduce the observed rms–flux relation and non-linear behaviour, which are more fundamental characteristics of the variability process than the power spectrum or spectral-timing properties. Models where X-ray variability is driven by accretion rate variations produced at different radii remain the most promising.
We study theoretical implications of a rapid very high energy (VHE) flare detected by MAGIC in the flat spectrum radio quasar
PKS 1222+216. The minimum distance from the jet origin at which this flare could be produced is 0.5 pc. A moderate Doppler
factor of the VHE source, , is allowed by all opacity constraints. The concurrent high-energy (HE) emission observed by Fermi provides estimates of the total jet power and the jet magnetic field strength. Energetic constraints for the VHE flare are
extremely tight: for an isotropic particle distribution, they require a huge comoving energy density in the emitting region
and a very efficient radiative process. We disfavour hadronic processes due to their low radiative efficiency, as well as
the synchrotron scenario recently proposed for the case of HE flares in the Crab nebula, since the parameters needed to overcome
the radiative losses are quite extreme. The VHE emission can be explained by the synchrotron self-Compton mechanism for or by the external radiation Compton mechanism involving the infrared radiation of the dusty torus for . After discussing several alternative scenarios, we propose that the extreme energy density constraint can be satisfied when
the emission comes from highly anisotropic short-lived bunches of particles formed by the kinetic beaming mechanism in magnetic
reconnection sites. By focusing the emitting particles into very narrow beams, this mechanism allows one to relax the causality
constraint on the source size, decreasing the required energy density by four orders of magnitude.
We study the relation between the mass accretion rate, the jet power and the black hole mass of blazars. With this aim, we
make use of the Sloan Digital Sky Survey and the 11-month catalogue of blazars detected at energies larger than 100 MeV by
the Large Area Telescope onboard the Fermi satellite. This allows us to construct a relatively large sample of blazars with information about both the luminosity (or
upper limits) of their emission lines (used as a proxy for the strength of the disc luminosity) and the luminosity of the
high-energy emission (used as a proxy for the jet power). We find a good correlation between the luminosity of the broad lines
and the γ-ray luminosities as detected by Fermi, both using the absolute values of the luminosities and normalizing them to the Eddington value. The data we have analysed
confirm that the division of blazars into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) is controlled
by the line luminosity in Eddington units. For small values of this ratio, the object is a BL Lac, while it is a FSRQ for
large values. The transition appears to be smooth, but a much larger number of objects is needed to confirm this point.
With the release of the first year Fermi catalogue, the number of blazars detected above 100 MeV lying at high redshift has been largely increased. There are 28 blazars at z>2 in the "clean" sample. All of them are Flat Spectrum Radio Quasars (FSRQs). We study and model their overall spectral energy distribution in order to find the physical parameters of the jet emitting region, and for all of them we estimate their black hole masses and accretion rates. We then compare the jet with the accretion disk properties, setting these sources in the broader context of all the other bright gamma-ray or hard X-ray blazars. We confirm that the jet power correlates with the accretion luminosity. We find that the high energy emission peak shifts to smaller frequencies as the observed luminosity increases, according to the blazar sequence, making the hard X-ray band the most suitable for searching the most luminous and distant blazars. Comment: 14 pages, 12 figures, accepted for publication in MNRAS
The extragalactic background light (EBL) is of fundamental importance both
for understanding the entire process of galaxy evolution and for gamma-ray
astronomy, but the overall spectrum of the EBL between 0.1-1000 microns has
never been determined directly from galaxy spectral energy distribution (SED)
observations over a wide redshift range. The evolving, overall spectrum of the
EBL is derived here utilizing a novel method based on observations only. This
is achieved from the observed evolution of the rest-frame K-band galaxy
luminosity function up to redshift 4 (Cirasuolo et al. 2010), combined with a
determination of galaxy SED-type fractions. These are based on fitting SWIRE
templates to a multiwavelength sample of about 6000 galaxies in the redshift
range from 0.2 to 1 from the All-wavelength Extended Groth Strip International
Survey (AEGIS). The changing fractions of quiescent galaxies, star-forming
galaxies, starburst galaxies and AGN galaxies in that redshift range are
estimated, and two alternative extrapolations of SED-types to higher redshifts
are considered. This allows calculation of the evolution of the luminosity
densities from the UV to the IR, the evolving star formation rate density of
the universe, the evolving contribution to the bolometric EBL from the
different galaxy populations including AGN galaxies and the buildup of the EBL.
Our EBL calculations are compared with those from a semi-analytic model, from
another observationally-based model and observational data. The EBL
uncertainties in our modeling based directly on the data are quantified, and
their consequences for attenuation of very high energy gamma-rays due to pair
production on the EBL are discussed. It is concluded that the EBL is well
constrained from the UV to the mid-IR, but independent efforts from infrared
and gamma-ray astronomy are needed in order to reduce the uncertainties in the
far-IR.
Spectra of the brightest blazars detected by the Fermi Gamma-ray Space Telescope Large Area Telescope cannot be described by a simple power law model. A much better description is obtained with a broken power law, with the break energies of a few GeV. We show here that the sharpness and the position of the breaks can be well reproduced by absorption of gamma-rays via photon--photon pair production on HeII Lyman recombination continuum and lines. This implies that the blazar zone lies inside the region of the highest ionization of the broad-line region (BLR) within a light-year from a super-massive black hole. The observations of gamma-ray spectral breaks open a way of studying the BLR photon field in the extreme-UV/soft X-rays, which are otherwise hidden from our view. Comment: 4 pages, 3 figures, 2 tables; published in ApJ Letters
The extragalactic background light (EBL) from the far infrared through the visible and extending into the ultraviolet is thought to be dominated by starlight, either through direct emission or through absorption and reradiation by dust. This is the most important energy range for absorbing \g-rays from distant sources such as blazars and gamma-ray bursts and producing electron positron pairs. In previous work we presented EBL models in the optical through ultraviolet by consistently taking into account the star formation rate (SFR), initial mass function (IMF) and dust extinction, and treating stars on the main sequence as blackbodies. This technique is extended to include post-main sequence stars and reprocessing of starlight by dust. In our simple model, the total energy absorbed by dust is assumed to be re-emitted as three blackbodies in the infrared, one at 40 K representing warm, large dust grains, one at 70 K representing hot, small dust grains, and one at 450 K representing polycyclic aromatic hydrocarbons. We find our best fit model combining the Hopkins and Beacom SFR using the Cole et al. parameterization with the Baldry and Glazebrook IMF agrees with available luminosity density data at a variety of redshifts. Our resulting EBL energy density is quite close to the lower limits from galaxy counts and in some cases below the lower limits, and agrees fairly well with other recent EBL models shortward of about 5 m. Deabsorbing TeV \g-ray spectra of various blazars with our EBL model gives results consistent with simple shock acceleration theory. We also find that the universe should be optically thin to \g-rays with energies less than 20 GeV. Comment: 13 pages, 10 figures, 3 tables, emulateapj. Version accepted by ApJ
The jets of powerful blazars propagate within regions relatively dense of radiation produced externally to the jet. This radiation is a key ingredient to understand the origin of the high energy emission of blazars, from the X-ray to the gamma-ray energy band. These external radiation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region, by assuming rather "standard" (i.e. "canonical") prescriptions for the accretion disk and its X-ray corona, the profile of the jet magnetic field and the external radiation. The magnetic energy density of a "canonical" jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad line region is particularly interesting, since it accounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X-ray spectrum. Furthermore it makes the high power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds. Comment: Revised version accepted for publication in MNRAS
The history of the development of statistical hypothesis testing in time series analysis is reviewed briefly and it is pointed out that the hypothesis testing procedure is not adequately defined as the procedure for statistical model identification. The classical maximum likelihood estimation procedure is reviewed and a new estimate minimum information theoretical criterion (AIC) estimate (MAICE) which is designed for the purpose of statistical identification is introduced. When there are several competing models the MAICE is defined by the model and the maximum likelihood estimates of the parameters which give the minimum of AIC defined by AIC = (-2)log-(maximum likelihood) + 2(number of independently adjusted parameters within the model). MAICE provides a versatile procedure for statistical model identification which is free from the ambiguities inherent in the application of conventional hypothesis testing procedure. The practical utility of MAICE in time series analysis is demonstrated with some numerical examples.
The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can work also for radio sources in general, and for blazar in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution as well as time dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations among the emission at different frequencies, specify the contribution of each region of the jet to the total emission, find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to qualitatively reproduce the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we explore the constraints which this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars. Comment: 12 pages, 10 postscript figures, submitted to MNRAS
We discuss the relation between the power carried by relativistic jets and the nuclear power provided by accretion, for a group of blazars including FSRQs and BL Lac objects. They are characterized by good quality broad band X-ray data provided by the Beppo SAX satellite. The jet powers are estimated using physical parameters determined from uniformly modelling their spectral energy distributions (SEDs). Our analysis indicates that for Flat Spectrum Radio Quasars the total jet power is of the same order as the accretion power. We suggest that blazar jets are likely powered by energy extraction from a rapidly spinning black hole through the magnetic field provided by the accretion disk. FSRQs must have large BH masses (10^8 - 10^9 solar masses) and high, near Eddington accretion rates. For BL Lac objects the jet luminosity is larger than the disk luminosity. This can be understood within the same scenario if BL Lac objects have masses similar to FSRQ but accrete at largely subcritical rates, whereby the accretion disk radiates inefficiently. Thus the ``unification'' of the two classes into a single blazar population, previously proposed on the basis of a spectral sequence governed by luminosity, finds a physical basis. Comment: 27 pages, 3 figures, ApJ in press
We describe a new method (HOP) for identifying groups of particles in N-body simulations. Having assigned to every particle an estimate of its local density, we associate each particle with the densest of the N_hop particles nearest to it. Repeating this process allows us to trace a path, within the particle set itself, from each particle in the direction of increasing density. The path ends when it reaches a particle that is its own densest neighbor; all particles reaching the same such particle are identified as a group. Combined with an adaptive smoothing kernel for finding the densities, this method is spatially adaptive, coordinate-free, and numerically straight-forward. One can proceed to process the output by truncating groups at a particular density contour and combining groups that share a (possibly different) density contour. While the resulting algorithm has several user-chosen parameters, we show that the results are insensitive to most of these, the exception being the outer density cutoff of the groups. Comment: LaTeX, 18 pages, 7 Postscript figures included. ApJ, in press. Source code available from http://www.sns.ias.edu/~eisenste/hop/hop.html