  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.

High Energy Astrophysical Phenomena