Fermi Gamma-ray Space Telescope

Fermi GBM observations during the V404 Cygni 2015 outburst

P. A. Jenke
C. A. Wilson-Hodge, M. S. Briggs, V. Connaughton, M. H. Finger, J. Homan, R. Remillard, E. Burns, P. Veres


On June 15, 2015, Fermi?s Gamma Ray Burst Monitor (GBM) and the Swift Burst Alert Telescope both triggered on a hard X-ray flare from a source that Swift identified as the black-hole binary V404 Cygni. Over the 13-day outburst, GBM triggered 169 times during 73 distinct flaring episodes. After decades in quiescence following the 1989 outburst observed by Ginga, V404 Cygni reached a brightness of 30 Crab with emission to 300 keV. With GBM?s 8-steradian field of view, we obtained time-resolved spectroscopy with a 57% duty cycle spanning the entire X-ray outburst. Above 8 keV, we trace Comptonized emission from hot electrons in the black hole?s corona, and the highly variable absorption, heavy at times, from surrounding ejected material. Multiple low frequency QPOs (0.1 Hz to 10 Hz) are seen throughout the outburst. We present hardness-intensity diagrams from Earth occultation measurements, showing that the source was in a hard state during its outburst.