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Register a new userConstraining the Location of Gamma-Ray Flares in Luminous Blazars
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Added by
Krzysztof Nalewajko
Publication date
2014
fields
Physics
and research's language is
English
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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.
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We present a gamma-ray photon flux and spectral variability study of the flat-spectrum radio quasar 3C 273 over a rapid flaring activity period between September 2009 to April 2010. Five major flares are observed in the source during this period. The most rapid flare observed in the source has a flux doubling time of 1.1 hr. The rapid gamma-ray flares allow us to constrain the location and size of the gamma-ray emission region in the source. The gamma gamma-opacity constrains the Doppler factor, $delta_{gamma} geq$ 10 for the highest energy (15 GeV) photon observed by the {it Fermi}-Large Area Telescope (LAT). Causality arguments constrain the size of the emission region to 1.6$times 10^{15}$ cm. The gamma-ray spectra measured over this period show clear deviations from a simple power law with a break in 1-2 GeV energy range. We discuss possible explanations for the origin of the gamma-ray spectral breaks. Our study suggests that the gamma-ray emission region in 3C 273 is located within the broad line region ($<$1.6 pc). The spectral behavior and temporal characteristics of the individual flares indicate the presence of multiple shock scenarios at the base of the jet.
I present a systematic study of gamma-ray flares in blazars. For this purpose, I propose a very simple and practical definition of a flare as a period of time, associated with a given flux peak, during which the flux is above half of the peak flux. I select a sample of 40 brightest gamma-ray flares observed by Fermi/LAT during the first 4 years of its mission. The sample is dominated by 4 blazars: 3C 454.3, PKS 1510-089, PKS 1222+216 and 3C 273. For each flare, I calculate a light curve and variations of the photon index. For the whole sample, I study the distributions of the peak flux, peak luminosity, duration, time asymmetry, average photon index and photon index scatter. I find that: 1) flares produced by 3C 454.3 are longer and have more complex light curves than those produced by other blazars; 2) flares shorter than 1.5 days in the source frame tend to be time-asymmetric with the flux peak preceding the flare midpoint. These differences can be largely attributed to a smaller viewing angle of 3C 454.3 as compared to other blazars. Intrinsically, the gamma-ray emitting regions in blazar jets may be structured and consist of several domains. I find no regularity in the spectral gamma-ray variations of flaring blazars.
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.
Blazar spectral models generally have numerous unconstrained parameters, leading to ambiguous values for physical properties like Doppler factor delta or fluid magnetic field B. To help remedy this problem, a few modifications of the standard leptonic blazar jet scenario are considered. First, a log-parabola function for the electron distribution is used. Second, analytic expressions relating energy loss and kinematics to blazar luminosity and variability, written in terms of equipartition parameters, imply delta, B, and the principal electron Lorentz factor gamma_pk. The external radiation field in a blazar is approximated by Ly alpha radiation from the broad line region (BLR) and ~0.1 eV infrared radiation from a dusty torus. When used to model 3C 279 SEDs from 2008 and 2009 reported by Hayashida et al. (2012), we derive delta ~ 20-30, B ~ few G, and total (IR + BLR) external radiation field energy densities u ~ 0.01 - 0.001 erg/cm^3, implying an origin of the gamma-ray emission site in 3C 279 at the outer edges of the BLR. This is consistent with the gamma-ray emission site being located at a distance R <~ Gamma^2 c t_{var} ~ 0.1 (Gamma/30)^2 (t_{var}/10^4 s) pc from the black hole powering 3C 279s jets, where t_{var} is the variability time scale of the radiation in the source frame, and at farther distances for narrow-jet and magnetic_reconnection models. Excess >~ 5 GeV gamma-ray emission observed with Fermi LAT from 3C 279 challenge the model, opening the possibility of hadronic origins of the emission. For low hadronic content, absolute jet powers of ~10% of the Eddington luminosity are calculated.
We present centimeter-band total flux density and linear polarization light curves illustrating the signature of shocks during radio band outbursts associated in time with gamma-ray flares detected by the Fermi LAT. The general characteristics of the spectral evolution during these events is well-explained by new radiative transfer simulations incorporating propagating oblique shocks and assuming an initially turbulent magnetic field. This finding supports the idea that oblique shocks in the jet are a viable explanation for activity from the radio to the gamma-ray band in at least some gamma-ray flares.
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