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

Markus Ackermann R. Anantua K. Asano 1 Luca Baldini Guido Barbiellini D. Bastieri J. Becerra Gonzalez R. Bellazzini E. Bissaldi R. D. Blandford E. D. Bloom R. Bonino E. Bottacini P. Bruel R. Buehler G. A. Caliandro R. A. Cameron M. Caragiulo P. A. Caraveo E. Cavazzuti C. Cecchi 2 C. C. Cheung 3 J. Chiang G. Chiaro S. Ciprini J. Cohen-Tanugi F. Costanza S. Cutini F. D’ammando F. De Palma R. Desiante S. W. Digel 4 N. Di Lalla 5 M. Di Mauro 6 L. Di Venere P. S. Drell C. Favuzzi S. J. Fegan E. C. Ferrara 7 Y. Fukazawa S. Funk 8 P. Fusco 9 F. Gargano D. Gasparrini N. Giglietto F. Giordano M. Giroletti 10 I. A. Grenier 11, 12 Lucas Guillemot 13, 14 S. Guiriec M. Hayashida E. Hays D. Horan 15 G. Jóhannesson S. Kensei D. Kocevski M. Kuss 16 G. La Mura 17 S. Larsson 18 L. Latronico J. Li 19 F. Longo F. Loparco 20 B. Lott 21 M. N. Lovellette P. Lubrano G. M. Madejski J. D. Magill S. Maldera A. Manfreda M. Mayer 22 M. N. Mazziotta P. F. Michelson 23 N. Mirabal T. Mizuno M. E. Monzani A. Morselli I. V. Moskalenko K. Nalewajko M. Negro 24, 25 E. Nuss 20 T. Ohsugi E. Orlando D. Paneque J. S. Perkins 26 M. Pesce-Rollins F. Piron 20 G. Pivato T. A. Porter 27 G. Principe R. Rando M. Razzano S. Razzaque A. Reimer J. D. Scargle C. Sgrò 28 M. Sikora D. Simone 29 E. J. Siskind F. Spada P. Spinelli 30 L. Stawarz J. B. Thayer D. J. Thompson 31 D. F. Torres E. Troja Y. Uchiyama 32 Y. Yuan 33 S. Zimmer 34
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 $\sim3.6\times10^{-5}\;{\rm photons}\;{\rm cm}^{-2}\;{\rm s}^{-1}$ averaged over orbital period intervals. It is the historically highest $\gamma$-ray flux observed from the source including past EGRET observations, with the $\gamma$-ray isotropic luminosity reaching $\sim10^{49}\;{\rm erg}\;{\rm s}^{-1}$. 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 ($\Gamma$) of 35 is necessary to avoid both internal $\gamma$-ray absorption and super-Eddington jet power. In the standard external-radiation-Comptonization scenario, $\Gamma$ should be at least 50 to avoid overproducing the synchrotron-self-Compton component. However, this predicts extremely low magnetization ($\sim5\times10^{-4}$). Equipartition requires $\Gamma$ as high as 120, unless the emitting region is a small fraction of the dissipation region. Alternatively, we consider $\gamma$ rays originating as synchrotron radiation of $\gamma_{\rm e}\sim1.6\times10^6$ electrons, in magnetic field $B\sim1.3$ 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 $\gamma$ rays in hadronic processes.
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Markus Ackermann, R. Anantua, K. Asano, Luca Baldini, Guido Barbiellini, et al.. Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June. The Astrophysical journal letters, Bristol : IOP Publishing, 2016, 824 (L20), 8 p. ⟨10.3847/2041-8205/824/2/L20⟩. ⟨insu-01360099⟩

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