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. Largeamplitude 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 Fopt ∝ F0.54 X 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.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
Hynes, R., Robinson, E., Terndrup, D., Gandhi, P., Froning, C., Wagner, R., Starrfield, S., Dhillon, V., & Marsh, T. (2019). Optical and X-ray correlations during the 2015 outburst of the black hole V404 Cyg. Monthly Notices of the Royal Astronomical Society, 487 (1), 60-78. https://doi.org/10.1093/mnras/stz1238