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

TL;DR

This paper reports on the detection of a bright gamma-ray flare from blazar 3C 279 in June 2015 using AGILE and Fermi-LAT, combined with multifrequency observations, revealing extreme Compton dominance and challenging existing leptonic models.

Contribution

It provides the first simultaneous multi-wavelength analysis of the June 2015 flare, highlighting high Compton dominance and constraints on the accretion disk luminosity, challenging one-zone leptonic models.

Findings

Gamma-ray to optical emission ratio increased threefold in hours.

Observed Compton dominance of approximately 100.

Flare behavior challenges standard leptonic emission models.

Abstract

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 June 2015. We use AGILE-GRID and Fermi-LAT $\gamma$-ray data, 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 distribution during the $\gamma$-ray flare. The $\gamma$-ray flaring data, compared with as yet unpublished simultaneous optical data which allow to set constraints on the big blue bump disk luminosity, show very high Compton dominance values of $\sim 100$, with a ratio of $\gamma$-ray to optical emission rising by a factor of three in a few hours. The multi-wavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the June 2015 flare are also compared with already published data and non-simultaneous historical 3C 279 archival data.