V404 Cyg with NuSTAR: relativistic reflection, jets and spin

In summer 2015 the Galactic LMXB V404 Cyg, one of the closest known black hole binary systems, went through its first major outburst in ~25 years, triggering a massive multiwavelength monitoring campaign to cover this remarkable event. In this talk, I will discuss results from the NuSTAR contribution to this campaign, focusing in particular on the data obtained during the height of the outburst activity. These data reveal a variety of extreme behaviour, from intense ~Eddington level flares to strong relativistic reflection. The contemporaneous onset of radio emission strongly suggests that this X-ray flaring is related to jet activity, with the ejected plasma/ base of the jet becoming the source of illumination of the disc. We are able to use the reflection observed to place constraints on the system geometry during these events, finding that the X-ray emitting regions in the jet are located very close to the black hole. In addition, our reflection analysis also allows us to place the first constraints on the black hole spin, and we find that V404 Cyg likely hosts a rapidly rotating black hole.