Optical and X-ray correlations during the 2015 outburst of the black hole V404 Cyg. Issue 1 (8th May 2019)

• Record Type: Journal Article
• Authors: Hynes, R I; Robinson, E L; Terndrup, D M; Gandhi, P; Froning, C S; Wagner, R M; Starrfield, S; Dhillon, V S; Marsh, T R
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 487:Issue 1(2019)
• Page Start: 60
• Page End: 78
• Publication Date: 2019-05-08
• Subjects: accretion, accretion discs -- X-rays: binaries -- X-rays: individual: V404 Cyg; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1238
• Languages: English
• ISSNs: 0035-8711

Abstract

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 time-scales or jet ejections. Rapid optical flaring on time-scales of seconds or less is incompatible with binary light-travel time-scales 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 lower envelope traces out a$F_{\rm opt} \propto F_{\rm X}^{0.54}$ relation that 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 anticorrelation 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.