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We present observations of rapid (sub-second) optical flux variability inV404 Cyg during its 2015 June outburst. Simultaneous three-band observationswith the ULTRACAM fast imager on four nights show steep power spectra dominatedby slow variations on ~100-1000s timescales. Near the peak of the outburst onJune 26, a dramatic change occurs and additional, persistent sub-second opticalflaring appears close in time to giant radio and X-ray flaring. The flaresreach peak optical luminosities of ~few x 10^{36} erg/s. Some are unresolveddown to a time resolution of 24 milliseconds. Whereas the fast flares arestronger in the red, the slow variations are bluer when brighter. The redderslopes, emitted power, and characteristic timescales of the fast flares can beexplained as optically-thin synchrotron emission from a compact jet arising onsize scales ~140-500 Gravitational radii (with a possible additionalcontribution by a thermal particle distribution). The origin of the slowervariations is unclear. The optical continuum spectral slopes are stronglyaffected by dereddening uncertainties and contamination by strong H{alpha}emission, but the variations of these slopes follow relatively stable loci as afunction of flux. Cross-correlating the slow variations between the differentbands shows asymmetries on all nights consistent with a small red skew (i.e.,red lag). X-ray reprocessing and non-thermal emission could both contribute tothese. These data reveal a complex mix of components over five decades intimescale during the outburst.

Furiously Fast and Red: Sub-second Optical Flaring in V404 Cyg during the 2015 Outburst Peak