Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June

LAT Collaboration

doi:10.3847/2041-8205/824/2/L20

paper

Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June

arXiv:1605.05324 · doi:10.3847/2041-8205/824/2/L20

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 $\sim 3.6 \times 1 0^{- 5} photons cm^{- 2} s^{- 1}$ averaged over orbital period intervals. It is the historically highest $γ$ -ray flux observed from the source including past EGRET observations, with the $γ$ -ray isotropic luminosity reaching $\sim 1 0^{49} erg s^{- 1}$ . During the outburst, the Fermi spacecraft, which has an orbital period of 95.4 min, 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-min binned timescales, with flux doubling times less than 5 min. 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 ( $\sim 5 \times 1 0^{- 4}$ ). 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 $γ_{e} \sim 1.6 \times 1 0^{6}$ electrons, in magnetic field $B \sim 1.3$ kG, accelerated by strong electric fields $E \sim B$ in the process of magnetoluminescence. At such short distance scales, one cannot immediately exclude production of $γ$ rays in hadronic processes.

arXiv comment8 pages, 4 figures. Accepted for publication in ApJ Letter. --Corresponding authors: Masaaki Hayashida (mahaya_at_icrr.u-tokyo.ac.jp), Greg Madejski (madejski_at_slac.stanford.edu), and Krzysztof Nalewajko (knalew_at_camk.edu.pl)

Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June

Abstract

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