# Optical and X-ray Correlations During the 2015 Outburst of the Black Hole V404 Cyg

## TL;DR

This study analyzes simultaneous optical and X-ray data during V404 Cyg's 2015 outburst, revealing correlations, rapid variability linked to jets, and a new relation between X-ray hardness and optical activity, driven by accretion changes.

## Contribution

It uncovers a novel correlation between X-ray hardness and optical variability, and demonstrates simultaneous jet-related rapid optical flaring and lagged correlations during an outburst.

## Key findings

- Optical variability correlates with X-ray emission with minimal lag.
- Rapid optical flares are associated with jet synchrotron emission.
- A new X-ray/optical brightness relation bounds the observed variability.

## 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 light-travel 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 lower-envelope traces out a Fopt proportional to Fx^0.54 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.