Correlated X-ray and optical variability in V404 Cygni in quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of greater than or similar to20 during a 60 ks observation. X-ray variations were well correlated with those in Ha, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Halpha flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outward to at least the circularization radius. The prompt response in the entire Ha line confirms that the variability is powered by X-ray (and/or EUV) irradiation.