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Register a new userEquipartition Gamma-Ray Blazars and the Location of the Gamma-Ray Emission Site in 3C 279
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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.
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Observations performed with the Fermi-LAT telescope have revealed the presence of a spectral break in the GeV spectrum of flat-spectrum radio quasars (FSRQs) and other low- and intermediate-synchrotron peaked blazars. We propose that this feature can be explained by Compton scattering of broad-line region (BLR) photons by a non-thermal population of electrons described by a log-parabolic function. We consider in particular a scenario in which the energy densities of particles, magnetic field, and soft photons in the emitting region are close to equipartition. We show that this model can satisfactorily account for the overall spectral energy distribution of the FSRQ 3C 454.3, reproducing the GeV spectal cutoff due to Klein-Nishina effects and a curving electron distribution.
In order to determine the location of the gamma-ray emission site in blazars, we investigate the time-domain relationship between their radio and gamma-ray emission. Light-curves for the brightest detected blazars from the first 3 years of the mission of the Fermi Gamma-ray Space Telescope are cross-correlated with 4 years of 15GHz observations from the OVRO 40-m monitoring program. The large sample and long light-curve duration enable us to carry out a statistically robust analysis of the significance of the cross-correlations, which is investigated using Monte Carlo simulations including the uneven sampling and noise properties of the light-curves. Modeling the light-curves as red noise processes with power-law power spectral densities, we find that only one of 41 sources with high quality data in both bands shows correlations with significance larger than 3-sigma (AO 0235+164), with only two more larger than even 2.25-sigma (PKS 1502+106 and B2 2308+34). Additionally, we find correlated variability in Mrk 421 when including a strong flare that occurred in July-September 2012. These results demonstrate very clearly the difficulty of measuring statistically robust multiwavelength correlations and the care needed when comparing light-curves even when many years of data are used. This should be a caution. In all four sources the radio variations lag the gamma-ray variations, suggesting that the gamma-ray emission originates upstream of the radio emission. Continuous simultaneous monitoring over a longer time period is required to obtain high significance levels in cross-correlations between gamma-ray and radio variability in most blazars.
Violent multi-wavelength variabilities are observed in gamma-ray-selected blazars. We present an analysis of long-term light curves for eight bright blazars to explore the co-variation pattern in the gamma-ray and radio bands. We extract their gamma-ray light curves and spectra with data observed by the Fermi/LAT since 2008. We find diverse co-variation patterns between the gamma-ray and radio (at 43 GHz) fluxes in these sources. The gamma-ray and radio fluxes of 3C 454.3 and PKS 1633+382 are correlated without any time-lag, suggesting that they are from the same radiation region. Similar correlation is also observed in 3C 273 and PKS 1222+216, but the radio flux is lag behind the gamma-ray flux approximately ~160 days and ~290 days, respectively. This likely suggests that their gamma-ray emission regions are located at the upstream of their radio cores at 43 GHz. The gamma-ray and radio fluxes of the other four blazars are not correlated, implying that the gamma-ray and radio emission may be from different regions in their jets. The gamma-ray light curves of the eight blazars can be decomposed into some long timescale variability components and fast spike flares. We propose that they may be attributed to the central engine activity and the magnetic reconnection process or turbulence in the local emission region, respectively.
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.
Bright and fast gamma-ray flares with hard spectra have been recently detected from the blazar 3C 279, with apparent GeV luminosities up to $10^{49}$ erg/s. The source is observed to flicker on timescales of minutes with no comparable optical-UV counterparts. Such observations challenge current models of high-energy emissions from 3C 279 and similar blazar sources that are dominated by relativistic jets along our line of sight with bulk Lorentz factors up to $ Gamma sim 20$ launched by supermassive black holes. We compute and discuss a model based on a clumpy jet comprising strings of compact plasmoids as indicated by radio observations. We follow the path of the synchrotron radiations emitted in the optical - UV bands by relativistic electrons accelerated around the plasmoids to isotropic Lorentz factors $gamma sim 1000$. These primary emissions are partly reflected back by a leading member in the string that acts as a moving mirror for the approaching companions. Around the plasmoids, shrinking emph{gap} transient overdensities of seed photons build up. These are upscattered into the GeV range by inverse Compton interactions with the relativistic electrons accelerated in situ. We show that such a combined process produces bright gamma-ray flares with minor optical to X-ray enhancements. Main features of our model include: bright gamma-ray flares with risetimes as short as a few minutes, occurring at distances of order $10^{18} $ cm from the central black hole; Compton dominance at GeV energies by factors up to some $10^2$; little reabsorption from local photon-photon interactions.
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