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Register a new userMeeting the Challenge from Bright and Fast Gamma-Ray Flares of 3C 279
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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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The optical polarization plane of some blazars occasionally exhibits smooth hundred degree long rotations. Multiple theoretical models have been proposed to explain the nature of such events. A deterministic origin of these rotations, however, remains uncertain. We aim to find repeating patterns of flares in gamma-ray light curves of blazars, which accompany optical polarization plane rotations. Such patterns have been predicted to occur by one of the models explaining this phenomenon. For the blazar 3C 279, where multiple polarization plane rotations have been reported in the literature, we obtain the Fermi-LAT gamma-ray light curve and analyze its intervals adjacent to polarization plane rotations. We find a complex characteristic pattern of flares in the gamma-ray light curve that is repeated during periods adjacent to three large amplitude EVPA rotation events in 3C 279. We discover a hidden EVPA rotation, which can only be seen in the relative Stokes parameters plane and that occurred simultaneously with the fourth repetition of the pattern. This finding strongly favors the hypothesis of emission features propagating in the jet as the reason of optical polarization plane rotations. Furthermore, it is compatible with the hypothesis of a sheath in the jet comprised of more slowly propagating emission features.
We report the AGILE detection and the results of the multifrequency follow-up observations of a bright $gamma$-ray flare of the blazar 3C 279 in June 2015. We use AGILE-GRID and Fermi-LAT $gamma$-ray data, together with Swift-XRT, Swift-UVOT, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the $gamma$-ray flare. The $gamma$-ray flaring data, compared with as yet unpublished simultaneous optical data which allow to set constraints on the big blue bump disk luminosity, show very high Compton dominance values of $sim 100$, with a ratio of $gamma$-ray to optical emission rising by a factor of three in a few hours. The multi-wavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the June 2015 flare are also compared with already published data and non-simultaneous historical 3C 279 archival data.
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.
GRB 131108A is a bright long Gamma-Ray Burst (GRB) detected by the Large Area Telescope and the Gamma-ray Burst Monitor on board the textit{Fermi Gamma-ray Space Telescope}. Dedicated temporal and spectral analyses reveal three $gamma$-ray flares dominating above 100 MeV, which are not directly related to the prompt emission in the GBM band (10 keV--10 MeV). The high-energy light curve of GRB 131108A (100 MeV -- 10 GeV) shows an unusual evolution: a steep decay, followed by three flares with an underlying emission, and then a long-lasting decay phase. The detailed analysis of the $gamma$-ray flares finds that the three flares are 6 -- 20 times brighter than the underlying emission and are similar to each other. The fluence of each flare, (1.6 $sim$ 2.0) $times$ 10$^{-6}$ erg cm$^{-2}$, is comparable to that of emission during the steep decay phase, 1.7 $times$ 10$^{-6}$ erg cm$^{-2}$. The total fluence from three $gamma$-ray flares is 5.3 $times$ 10$^{-6}$ erg cm$^{-2}$. The three $gamma$-ray flares show properties similar to the usual X-ray flares that are sharp flux increases, occurring in $sim$ 50% of afterglows, in some cases well after the prompt emission. Also, the temporal and spectral indices during the early steep decay phase and the decaying phase of each flare show the consistency with a relation of the curvature effect ($hat{alpha}$ = 2 + $hat{beta}$), which is the first observational evidence of the high-latitude emission in the GeV energy band.
The shorter-than-Schwarzschild 3C 279 variability flare on June 2015 is very puzzling. Its nature cannot be due to any NS merging nor to a medium sized (hundred million solar mass) BH collapse. Our preliminary model is based on the long-life (a third of a year) merging of a medium size BH (hundred of solar mass) jet spiralling toward the largest AGN one, (billion solar mass), that is dragging by tidal torques the medium small size BH jet along the main AGN 3C 279 one. The tidal torque is aligning both jets toward Earth. The twin overlapping blazars may offer at once a long and a short scale variability consistent with the surprising Fermi discovers.
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