High-energy gamma-ray observations of the accreting black hole V404 Cygni during its 2015 June outburst

We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in June–July 2015. Detailed analyses reveal a possible excess of γ-ray emission on 26 June 2015, with a very soft spectrum above 100 MeV, at a position consistent with the direction of V404 Cyg (within the 95% confidence region and a chance probability of 4×10 −4). This emission cannot be associated with any previously-known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the lightcurve of V404 Cyg, at the end of the main active phase of its outburst, strengthens the association with V404 Cyg. If the γ-ray emission is associated with V404 Cyg, the simultaneous detection of 511 keV annihilation emission by INTEGRAL requires that the high-energy γ rays originate away from the corona, possibly in a Blandford-Znajek jet. The data give support to models involving a magnetically-arrested disk where a bright γ-ray jet can reform after the occurrence of a major transient ejection seen in the radio.