The Bright gamma-ray Flare of 3C 279 in 2015 June 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 flare of the blazar 3C 279 in 2015 June. We use AGILE and Fermi gamma-ray data, together with Swift X-ray andoptical-ultraviolet data, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the gamma-ray flare. The gamma-ray flaring data, compared with as yet unpublished simultaneous optical data that will allow constraints on the big blue bump disk luminosity, show very high Compton dominance values of ~100, with the ratio of gamma-ray to optical emission rising by a factor of three in a few hours. The multiwavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the 2015 June flare are also compared with already published data and nonsimultaneous historical 3C 279 archival data.