Gamma-ray and Optical properties of the flat spectrum radio quasar 3C 279 flare in June 2015

Abstract

The flat spectrum radio quasar 3C 279 was observed in an extremely high activity state on June 16, 2015 (MJD 57189). In this paper, we investigate the properties of this flaring episode in the high energy $\gamma$-ray and optical bands using data from the \emph{Fermi}-LAT, SMARTS, and SPOL observations during the period June 1-30, 2015 (MJD 57174-57203). The highest emission state in the $\gamma$-ray band detected by the \emph{Fermi}-LAT exhibits a peak flux of $\sim$ 2$\times$10$^{-7}$ erg~cm$^{-2}$~s$^{-1}$ which is more than 25 times the flux level measured in the low activity state of the source. The temporal analysis of the daily \emph{Fermi}-LAT light curve suggests that the giant flaring episode has characteristic rise and decay times less than one day. The optical daily light curves in B, V, R, and J bands also indicate the flaring activity from 3C 279 with flux levels peaking for two days on June 16-17, 2015 (MJD 57189-57190). The discrete correlation function analysis indicates a time lag of 1 day or longer between the $\gamma$-ray and optical peaks during the flaring episode. The $\gamma$-ray emission is also observed to show a harder-when-brighter behaviour whereas optical emission exhibits an opposite behaviour. The $\gamma$-ray emission region during the flare is observed to be very compact and is located close to the base of the jet. The degree of linear polarization in the wavelength range 500-700 nm measured using SPOL during this period is also highly variable with a peak of $\sim$ 30$\%$ one day after the $\gamma$-ray flare. Near simultaneous $\gamma$-ray flux points show a linear anti-correlation with the degree of polarization during the period of $\gamma$-ray flare. The significant drop in the degree of linear polarization suggests a sudden increase in the tangled magnetic field strength in the emission region.