Swift UVOT observations of the 2015 outburst of V404 Cygni

The black-hole binary, V404 Cyg, went into outburst in June 2015, after 26 years of X-ray quiescence. We observed the outburst with the Neil Gehrels Swift observatory. We present the optical/UV observations taken with the Swift Ultra-Violet Optical Telescope, and compare them with the X-ray observations obtained with the Swift X-ray telescope. We find that dust extinction, affecting the optical/UV, does not correlate with absorption due to neutral hydrogen, which affects the X-ray emission. We suggest that there is a small inhomogeneous high density absorber, containing a negligible amount of dust, close to the black hole. Overall, temporal variations in the optical/UV emission trace those in the X-ray band, with a time-lag of (0 - 30)s, with the optical/UV generally lagging the X-rays. The amplitude of the variations is correlated with flux in both bands, but less significantly in the optical/UV. The variability in the light curves may be produced by a complex combination of processes. Some of the variability observed only in the X-ray may be due to the presence of a local, inhomogeneous and dust-free absorber. While variability visible in both the X-ray and optical/UV bands may instead be driven by the accretion flow: the X-rays are produced in the inner accretion disc, some of which are reprocessed to the optical/UV; and/or the X-ray and optical/UV emission is produced within the jet.