The black-hole binary , V404 Cygni , 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 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 that affects the X-ray emission .
We suggest 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 appear to trace those in the 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 correlates with flux , but this correlation is less significant in the optical/UV .
The variability in the light curves may be produced by a complex combination of processes .
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 the X-ray and optical/UV 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 .