Optical and X-ray correlations during the 2015 outburst of the black hole V404 Cyg

Abstract

We present a serendipitous multiwavelength campaign of optical photometry simultaneous with Integral X-ray monitoring of the 2015 outburst of the black hole V404 Cyg. Large amplitude optical variability is generally correlated with X-rays, with lags of order a minute or less compatible with binary light travel timescales or jet ejections. Rapid optical flaring on time-scales of seconds or less is incompatible with binary lighttravel timescales and has instead been associated with synchrotron emission from a jet. Both this rapid jet response and the lagged and smeared one can be present simultaneously. The optical brightness is not uniquely determined by the X-ray brightness, but the X-ray/optical relationship is bounded by a lower-envelope such that at any given optical brightness there is a maximum X-ray brightness seen. This lowerenvelope traces out a F opt ∝ F 0.54 X relation which can be approximately extrapolated back to quiescence. Rapid optical variability is only seen near this envelope, and these periods correspond to the hardest hard X-ray colours. This correlation between hard X-ray colour and optical variability (and anti-correlation with optical brightness) is a novel finding of this campaign, and apparently a facet of the outburst behaviour in V404 Cyg. It is likely that these correlations are driven by changes in the central accretion rate and geometry.

The INTEGRAL view on Black Hole X-ray Binaries

“…almost the exact same variations were observed ), and 3) flares during which enhanced optical and soft X-ray emissions preceded a hard X-ray flare [209]. Moreover, simultaneous NuSTAR and optical observations taken on sub-second time scales showed that the optical emission lagged the X-rays by roughly 100ms [142,186]. This lag was interpreted as the delay between fluctuations generated in the hot corona around the BH, that propagate up to the relativistic jet base about 1000 R g away, which is responsible for part of the optical emission [e.g., 266].…”

The obscured X-ray binaries V404 Cyg, Cyg X–3, V4641 Sgr, and GRS 1915+105

“…The two X-ray outbursts in 1989 and 2015 were both hard-state outbursts with no excursion into a soft X-ray state, although the luminosities reached or exceeded the Eddington luminosity. During the 2015 outbursts, a plethora of X-ray spectral behavior was observed from V404 Cyg, with some applicable to absorption events with intermediate flux densities and X-ray spectra not consistent with a Comptonization model, and some to intrinsic variations with very high or low flux densities consistent with a Comptonization model (Motta et al 2017a,b;Sánchez-Fernández et al 2017;Kajava et al 2018;Walton et al 2017;Hynes et al 2019). In both cases, the X-ray spectra exhibit fast changes from one state to another in a matter of seconds to minutes (Motta et al 2017a;Kajava et al 2018;Sánchez-Fernández et al 2017;Walton et al 2017).…”

Swift UVOT observations of the 2015 outburst of V404 Cygni

“…For two segments the behaviour differs from these two categories: in the T0+877 ks segment the Xray and u-band count rates change independently producing vertical and horizontal lines, while in the T0+820 ks segment similar behaviour is observed, but a hysteresis-like pattern is produced. Large variations in X-ray flux, but no pronounced optical response was also reported by Hynes et al (2019).…”

“…S.2. This horizontal behaviour could suggest that either the X-ray and optical/UV emission are produced independent or that the optical/UV emitting region is unable to respond to the increase in the X-rays (see also Hynes et al 2019). However, the optical/UV count rate versus X-ray count rate figure of the T0+877 segment also displays a vertical line, which shows the optical/UV count rate changing while the X-ray count rate remaining constant.…”