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Register a new userAbsorption effects in the blazars gamma-ray spectra due to luminous stars crossing the jet
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Gamma-ray emission in active galaxies is likely produced within the inner jet, or in the close vicinity of the supermassive black hole (SMBH) at sub-parsec distances. Gamma rays have to pass through the surrounding massive stellar cluster which luminous stars can accidentally appear close to the observers line of sight. In such a case, soft radiation of massive stars can create enough target for transient absorption of the gamma rays in multi-GeV to TeV energy range. We consider the effect of such stellar encounters on the gamma-ray spectrum produced within the massive stellar cluster surrounding a central SMBH. We predict characteristic, time-dependent effects on the gamma-ray spectra due to the encounter with the single luminous star and also stellar binary system. We conclude that during the encounter, the gamma-ray spectrum of an active galaxy should steepen at tens of GeV and harden in the range of hundreds of GeV. As an example, we consider such effects on the spectra observed from a typical blazar, 1ES 1959+650 (in an active state) and also in the case of a radio galaxy M87 (in a low state). It is shown that observation of such transient characteristic features in the gamma-ray spectra, observed from blazars and radio galaxies, lays within the sensitivity of the future Cherenkov Telescope Array.
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Gamma-ray luminosities of some quasar-associated blazars imply jet powers reaching values comparable to the accretion power even if assuming very strong Doppler boosting and very high efficiency of gamma-ray production. With much lower radiative efficiencies of protons than of electrons, and the recent reports of very strong coupling of electrons with shock-heated protons indicated by Particle-in-Cell (PIC) simulations, the leptonic models seem to be strongly favored over the hadronic ones. However, the electron-proton coupling combined with the ERC (External-Radiation-Compton) models of gamma-ray production in leptonic models predict extremely hard X-ray spectra, with energy indices about 0. This is inconsistent with the observed 2-10 keV slopes of blazars, which cluster around an index value of 0.6. This problem can be resolved by assuming that electrons can be cooled down radiatively to non-relativistic energies, or that blazar spectra are entirely dominated by the SSC (Synchrotron-Self Compton) component up to at least 10 keV. Here, we show that the required cooling can be sufficiently efficient only at distances r < 0.03pc. SSC spectra, on the other hand, can be produced roughly co-spatially with the observed synchrotron and ERC components, which are most likely located roughly at a parsec scale. We show that the dominant SSC component can also be produced much further than the dominant synchrotron and ERC components, at distances larger than 10 parsecs. Hence, depending on the spatial distribution of the energy dissipation along the jet, one may expect to see gamma-ray/optical events with either correlated or uncorrelated X-rays. In all cases the number of electron-positron pairs per proton is predicted to be very low. The direct verification of the proposed SSC scenario requires sensitive observations in the hard X-ray band which is now possible with the NuSTAR satellite.
Locating the gamma-ray emission sites in blazar jets is a long-standing and highly controversial issue. We investigate jointly several constraints on the distance scale r and Lorentz factor Gamma of the gamma-ray emitting regions in luminous blazars (primarily flat spectrum radio quasars, FSRQs). Working in the framework of one-zone external radiation Comptonization (ERC) models, we perform a parameter space study for several representative cases of actual gamma-ray flares in their multiwavelength context. We find a particularly useful combination of three constraints: from an upper limit on the collimation parameter Gamma*theta <~ 1, from an upper limit on the synchrotron self-Compton (SSC) luminosity L_SSC <~ L_X, and from an upper limit on the efficient cooling photon energy E_cool,obs <~ 100 MeV. These three constraints are particularly strong for sources with low accretion disk luminosity L_d. The commonly used intrinsic pair-production opacity constraint on Gamma is usually much weaker than the SSC constraint. The SSC and cooling constraints provide a robust lower limit on the collimation parameter Gamma*theta >~ 0.1 - 0.7. Typical values of r corresponding to moderate values of Gamma ~ 20 are in the range 0.1 - 1 pc, and are determined primarily by the observed variability time scale t_var,obs. Alternative scenarios motivated by the observed gamma-ray/mm connection, in which gamma-ray flares of t_var,obs ~ a few days are located at r ~ 10 pc, are in conflict with both the SSC and cooling constraints. Moreover, we use a simple light travel time argument to point out that the gamma-ray/mm connection does not provide a significant constraint on the location of gamma-ray flares. We argue that spine-sheath models of the jet structure do not offer a plausible alternative to external radiation fields at large distances, however, an extended broad-line region is an idea worth exploring.
Blazars are strongly variable sources that occasionally show spectacular flares visible in various energy bands. These flares are often, but not always, correlated. In a number of cases the peaks of optical flares are found to be somewhat delayed with respect to the gamma-ray peaks. One notable example of such a delay was found in 3C 279 by Hayashida et al. and interpreted as a result of steeper drop with distance of the energy density of external radiation field than of the magnetic energy density. In this paper we demonstrate that in general, depending on the respective energy density profile along the jet, such lags can have both signs and that they can take place for any ratio of these energy densities. We study the dependence of such lags on the ratio of these energy densities at a distance of a maximal energy dissipation in a jet, on their gradients, as well as on the time profile of the relativistic electron injection within the moving source. We show how prominent such lags can be, and what are their expected time scales. We suggest that studies of such lags can provide a powerful tool to resolve the structure of relativistic jets and their radiative environment. As an example we model the lag observed in 3C 279, showing that in this object the flare is produced at a distance of a few parsecs from the central black hole, consistent with our previous inferences based on the spectra and optical polarization properties.
We investigate the spectral properties of the brightest gamma-ray flares of blazars detected by the Fermi Large Area Telescope. We search for the presence of spectral breaks and measure the spectral curvature on typical time scales of a few days. We identify significant spectral breaks in fewer than half of the analyzed flares, but their parameters do not show any discernible regularities, and in particular there is no indication for gamma-ray absorption at any fixed source-frame photon energy. More interestingly, we find that the studied blazars are characterized by significant spectral variability. Gamma-ray flares of short duration are often characterized by strong spectral curvature, with the spectral peak located above 100 MeV. Since these spectral variations are observed despite excellent photon statistics, they must reflect temporal fluctuations in the energy distributions of the emitting particles. We suggest that highly regular gamma-ray spectra of blazars integrated over long time scales emerge from a superposition of many short-lived irregular components with relatively narrow spectra. This would imply that the emitting particles are accelerated in strongly turbulent environments.
The highest-energy blazars exhibit non-thermal radiation extending beyond 1 TeV with high luminosities and strong variabilities, indicating extreme particle acceleration in their relativistic jets. The gamma-ray spectra of blazars contain information about the distribution and cooling processes of high-energy particles in jets, the extragalactic background light between the source and the observer, and potentially, the environment of the gamma-ray emitting region and exotic physics that may modify the opacity of the universe to gamma rays. We use data from Fermi-LAT and VERITAS to study the variability and spectra of a sample of TeV blazars across a wide range of gamma-ray energies, taking advantage of more than ten years of data from both instruments. The variability in both the GeV and TeV gamma-ray bands is investigated using a Bayesian blocks method to identify periods with a steady flux, during which the average gamma-ray spectra, after correcting for the pair absorption effect from propagation, can be parameterized without the risk of mixing different flux states. We report on the search for intrinsic spectral curvature and spectral variability in these blazars, in an effort to understand the physical mechanisms behind the high-energy gamma-ray spectra of TeV blazars.
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