Fermi-Large Area Telescope Observations of the Exceptional Gamma-ray Flare from 3C 279 in 2015 June

TL;DR

This paper reports on the unprecedented gamma-ray flare from 3C 279 observed by Fermi-LAT in June 2015, revealing the highest flux ever detected from this source and providing insights into the emission region and spectral characteristics.

Contribution

It presents the first detailed analysis of the 2015 June gamma-ray flare from 3C 279, including flux measurements, spectral features, and implications for the emission region's location and Doppler factor.

Findings

Highest gamma-ray flux from 3C 279 recorded

Spectral break/curvature observed during peak activity

Emission region likely at the outer edge of the broad line region

Abstract

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