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The flat spectrum radio quasar 3C 279 was observed in an extremely highactivity state on June 16, 2015 (MJD 57189). In this paper, we investigate theproperties of this flaring episode in the high energy $\gamma$-ray and opticalbands using data from the \emph{Fermi}-LAT, SMARTS, and SPOL observationsduring the period June 1-30, 2015 (MJD 57174-57203). The highest emission statein the $\gamma$-ray band detected by the \emph{Fermi}-LAT exhibits a peak fluxof $\sim$ 2$\times$10$^{-7}$ erg~cm$^{-2}$~s$^{-1}$ which is more than 25 timesthe flux level measured in the low activity state of the source. The temporalanalysis of the daily \emph{Fermi}-LAT light curve suggests that the giantflaring episode has characteristic rise and decay times less than one day. Theoptical daily light curves in B, V, R, and J bands also indicate the flaringactivity 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 timelag of 1 day or longer between the $\gamma$-ray and optical peaks during theflaring episode. The $\gamma$-ray emission is also observed to show aharder-when-brighter behaviour whereas optical emission exhibits an oppositebehaviour. The $\gamma$-ray emission region during the flare is observed to bevery compact and is located close to the base of the jet. The degree of linearpolarization in the wavelength range 500-700 nm measured using SPOL during thisperiod 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 linearanti-correlation with the degree of polarization during the period of$\gamma$-ray flare. The significant drop in the degree of linear polarizationsuggests a sudden increase in the tangled magnetic field strength in theemission region.

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