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 2015 June-July. Detailed analyses reveal a possible excess of γ -ray emission on 2015 26 June, with a very soft spectrum above 100 MeV, at a position consistent with the direction of V404 Cyg (within the 95 per cent 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 light curve 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 reqires 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 disc where a bright γ -ray jet can re-form after the occurrence of a major transient ejection seen in the radio.