MINUTE-TIMESCALE >100 MeV γ-RAY VARIABILITY DURING THE GIANT OUTBURST OF QUASAR 3C 279 OBSERVED BY FERMI-LAT IN 2015 JUNE

Abstract

On 2015 June 16, Fermi-LAT observed a giant outburst from the flat spectrum radio quasar 3C 279 with a peak >100 MeV flux of ∼3.6 × 10 −5 photons cm −2 s −1 , averaged over orbital period intervals. It is historically the highest γ-ray flux observed from the source, including past EGRET observations, with the γ-ray isotropic luminosity reaching ∼10 49 erg s −1. During the outburst, the Fermi spacecraft, which has an orbital period of 95.4 minutes, was operated in a special pointing mode to optimize the exposure for 3C 279. For the first time, significant flux variability at sub-orbital timescales was found in blazar observations by Fermi-LAT. The source flux variability was resolved down to 2-minute binned timescales, with flux doubling times of less than 5 minutes. The observed minute-scale variability suggests a very compact emission region at hundreds of Schwarzschild radii from the central engine in conical jet models. A minimum bulk jet Lorentz factor (Γ) of 35 is necessary to avoid both internal γ-ray absorption and super-Eddington jet power. In the standard external radiation Comptonization scenario, Γ should be at least 50 to avoid overproducing the synchrotron self-Compton component. However, this predicts extremely low magnetization (∼5 × 10 −4). Equipartition requires Γ as high as 120, unless the emitting region is a small fraction of the dissipation region. Alternatively, we consider γ rays originating as synchrotron radiation of γ e ∼ 1.6 × 10 6 electrons, in a magnetic field B ∼ 1.3 kG, accelerated by strong electric fields E ∼ B in the process of magnetoluminescence. At such short distance scales, one cannot immediately exclude the production of γ-rays in hadronic processes.

Blazar variability power spectra from radio up to TeV photon energies: Mrk 421 and PKS 2155−304

“…As a class, blazars are known to display strong vari-ability ranging from radio to VHE γ−ray energies and on multiple timescales (e.g., Aller et al 1999;Wagner & Witzel 1995;Falomo et al 2014;Abdo et al 2010) with a factor of few intensity changes on timescales as short as minutes which are found be especially prominent at high energies (Cui 2004;Aharonian et al 2007;Albert et al 2007;Ackermann et al 2016;Zhu et al 2018). Highly efficient particle acceleration, either due to the formation of shocks and turbulence in the jet flow (Agudo et al 2018;Marscher 2014;Hughes et al 2011), or annihilation of the magnetic field lines at the magnetic reconnection sites (e.g., Giannios 2013;Sironi et al 2015;Petropoulou et al 2016) are considered to be prime candidates for the energy dissipation processes.…”

Effects of Spin on High-Energy Radiation From Accreting Black Holes

“…Although BL Lacs (and blazars in general) have jets roughly aligned with the observer, at small radii there could be a misalignment between the jet and spin axes (see Section 3.5). Such a misalignment could significantly enhance the high-energy emission from close to the black hole, leading to the intriguing possibility that near horizon emission is responsible for the short-timescale variability observed in these systems (e.g., Aharonian et al 2007;Albert et al 2007;Aleksić et al 2011;Ackermann et al 2016).…”

e-ASTROGAM mission: a major step forward for gamma-ray polarimetry

“…22,28). In that case, the polarization of gamma-ray emission during such outbursts should be very strong and readily measurable by e-ASTROGAM.…”