Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June: Ammando 24,25 , F. de Palma 12

• Published in: Astrophysical journal. Letters 2016-06, Vol.824 (L20)
• Main Authors: Ackermann, Markus, Anantua, R., Asano, K., Baldini, Luca, Barbiellini, Guido, Bastieri, D., Gonzalez, J. Becerra, Bellazzini, R., Bissaldi, E., Blandford, R. D., Bloom, E. D., Bonino, R., Bottacini, E., Bruel, P., Buehler, R., Caliandro, G. A., Cameron, R. A., Caragiulo, M., Caraveo, P. A., Cavazzuti, E., Cecchi, C., Cheung, C. C., Chiang, J., Chiaro, G., Ciprini, S., Cohen-Tanugi, J., Costanza, F., Cutini, S., D’ammando, F., Palma, F. De, Desiante, R., Digel, S. W., Lalla, N. Di, Mauro, M. Di, Venere, L. Di, Drell, P. S., Favuzzi, C., Fegan, S. J., Ferrara, E. C., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Giglietto, N., Giordano, F., Giroletti, M., Grenier, I. A., Guillemot, Lucas, Guiriec, S., Hayashida, M., Hays, E., Horan, D., Jóhannesson, G., Kensei, S., Kocevski, D., Kuss, M., Mura, G. La, Larsson, S., Latronico, L., Li, J., Longo, F., Loparco, F., Lott, B., Lovellette, M. N., Lubrano, P., Madejski, G. M., Magill, J. D., Maldera, S., Manfreda, A., Mayer, M., Mazziotta, M. N., Michelson, P. F., Mirabal, N., Mizuno, T., Monzani, M. E., Morselli, A., Moskalenko, I. V., Nalewajko, K., Negro, M., Nuss, E., Ohsugi, T., Orlando, E., Paneque, D., Perkins, J. S., Pesce-Rollins, M., Piron, F., Pivato, G., Porter, T. A., Principe, G., Rando, R., Razzano, M., Razzaque, S., Reimer, A., Scargle, J. D., Sgrò, C., Sikora, M., Simone, D., Siskind, E. J., Spada, F.
• Format: Article
• Language: English
• Subjects: Sciences of the Universe
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8205/824/2/L20

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.