Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak

We present observations of rapid (sub-second) optical flux variabilityin V404 Cyg during its 2015 June outburst. Simultaneous three-bandobservations with the ULTRACAM fast imager on four nights show steeppower spectra dominated by slow variations on ˜100-1000 stime-scales. Near the peak of the outburst on June 26, a dramatic changeoccurs and additional, persistent sub-second optical flaring appearsclose in time to giant radio and X-ray flaring. The flares reach peakoptical luminosities of ˜ few × 10³⁶ ergs^-1. Some are unresolved down to a time resolution of 24 ms.Whereas the fast flares are stronger in the red, the slow variations arebluer when brighter. The redder slopes, emitted power and characteristictime-scales of the fast flares can be explained as optically thinsynchrotron emission from a compact jet arising on size scales˜140-500 Gravitational radii (with a possible additionalcontribution by a thermal particle distribution). The origin of theslower variations is unclear. The optical continuum spectral slopes arestrongly affected by dereddening uncertainties and contamination bystrong Hα emission, but the variations of these slopes followrelatively stable loci as a function of flux. Cross-correlating the slowvariations between the different bands shows asymmetries on all nightsconsistent with a small red skew (I.e. red lag). X-ray reprocessing andnon-thermal emission could both contribute to these. These data reveal acomplex mix of components over five decades in time-scale during theoutburst.