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The black-hole binary, V404 Cygni, went into outburst in June 2015, after 26years of X-ray quiescence. We observed the outburst with the Neil Gehrels Swiftobservatory. We present optical/UV observations taken with the SwiftUltra-violet Optical Telescope, and compare them with the X-ray observationsobtained with the Swift X-ray Telescope. We find that dust extinction affectingthe optical/UV, does not correlate with absorption due to neutral hydrogen thataffects the X-ray emission. We suggest there is a small inhomogeneous highdensity absorber containing a negligible amount of dust, close to the blackhole. Overall, temporal variations in the optical/UV appear to trace those inthe X-rays. During some epochs we observe an optical time-lag of (15 - 35)s.For both the optical/UV and X-rays, the amplitude of the variations correlateswith flux, but this correlation is less significant in the optical/UV. Thevariability in the light curves may be produced by a complex combination ofprocesses. Some of the X-ray variability may be due to the presence of a local,inhomogeneous and dust-free absorber, while variability visible in both theX-ray and optical/UV may instead be driven by the accretion flow: the X-raysare produced in the inner accretion disc, some of which are reprocessed to theoptical/UV; and/or the X-ray and optical/UV emission is produced within thejet.

Swift UVOT observations of the 2015 outburst of V404 Cygni