Living on a Flare: Relativistic Reflection in V404 Cyg Observed by NuSTAR during Its Summer 2015 Outburst

Abstract

We present first results from a series of NuSTAR observations of the black hole X-ray binary404 obtained during its summer 2015 outburst, primarily focusing on observations during the height of this outburst activity. The NuSTAR data show extreme variability in both the flux and spectral properties of the source. This is partly driven by strong and variable line-of-sight absorption, similar to previous outbursts. The latter stages of this observation are dominated by strong flares, reaching luminosities close to Eddington. During these flares, the central source appears to be relatively unobscured and the data show clear evidence for a strong contribution from relativistic reflection, providing a means to probe the geometry of the innermost accretion flow. Based on the flare properties, analogy with other Galactic black hole binaries, and also the simultaneous onset of radio activity, we argue that this intense X-ray flaring is related to transient jet activity during which the ejected plasma is the primary source of illumination for the accretion disk. If this is the case, then our reflection modelling implies that these jets are launched in close proximity to the black hole (as close as a few gravitational radii), consistent with expectations for jet launching models that tap either the spin of the central black hole, or the very innermost accretion disk. Our analysis also allows us to place the first constraints on the black hole spin for this source, which we find to be a * > 0.92 (99% statistical uncertainty, based on an idealized lamppost geometry).

A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni

“…(18). In brief, the unfiltered event files were cleaned using NUPIPELINE, part of the NUSTARDAS (17) software package (using software version 1.5.1 and caldb version 20150316).…”

Giant Metrewave Radio Telescope Monitoring of the Black Hole X-Ray Binary, V404 Cygni during Its 2015 June Outburst

“…The black hole spin of V404 Cygni has recently been estimated to be a * > 0.92 via reflection modelling of NuStar X-ray data (Walton et al 2017) on the June 2015 outburst. While V404 Cygni's spin parameter estimate lies at the upper end of the distribution of known BHXB spin parameters, it can be seen from Fig.…”

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

“…The difference between the two epochs is striking, with a much harder spectrum in Epoch 2 and a jump in observed flux by a factor of 15. No absorption correction is performed since the source spectra are highly complex 6,7 . In all panels, the lower grey and upper red sets of SEDs correspond to Epochs 1 and 2 (pre--and post--transition), respectively.…”

“…Our focus here is timing analysis and we defer to ref. 6 for the spectral analysis of the NuSTAR data. For the broadband SEDs presented in Supplementary Fig.…”