Fermi-large area telescope observations of the exceptional gamma-ray flare from 3C 279 in 2015 June

An exceptional γ-ray outburst from 3C 279 was detected by the Fermi-Large Area Telescope (LAT) in 2015 June. In the energy range of 0.1–300 GeV, the highest flux measured is (39.1 ± 2.5) × 10⁻⁶ $ph{cm}^{-2}{s}^{-1}$, which is the highest γ-ray flux ever detected from 3C 279, exceeding the previous historically brightest flare observed by EGRET in 1996. The high activity period consists of three major flares with the last one being the brightest. All but one flare show a faster rise and slower decay pattern, and at the peak of the activity, the γ-ray spectrum is found to show a clear signature of break/curvature. The obtained spectral parameters hint that the peak of the inverse Compton emission lies in the LAT energy range (around ∼1 GeV), which is in contrast to that seen during the 2013 December and 2014 April γ-ray flares of 3C 279. From the γγ pair opacity arguments, the minimum Doppler factor is estimated to be 14, and the location of the γ-ray emitting region is found to be either at the outer edge of the broad line region or farther out from it.