The Bright $\gamma$-ray Flare of 3C 279 in June 2015: AGILE Detection and Multifrequency Follow-up Observations.

We report the AGILE detection and the results of the multifrequency follow-up observations of a bright $\ gamma$- ray flareof the blazar3C 279 in June 2015. We use AGILE-GRID and Fermi-LAT $\ gamma$- raydata, together with Swift-XRT, Swift-UVOT, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distributionduring the $\ gamma$- ray flare. The $\ gamma$- ray flaringdata, compared with as yet unpublished simultaneous optical data which allow to set constraintson the big blue bump disk luminosity, show very high Compton dominance values of $\sim 100$, with a ratio of $\ gamma$- rayto optical emission rising by a factor of three in a few hours. The multi-wavelength behavior of the source during the flarechallenges one-zone leptonic theoretical models. The new observations during the June 2015 flareare also compared with already published data and non-simultaneoushistorical 3C 279 archival data.