Violent Optical Variations and Correlation between Optical and X-ray Variability During the 2015 Outbursts in V404 Cygni

We performed the optical photometry of the two outbursts in 2015–2016 in V404 Cyg, a black-hole X-ray transient and investigated the connection between our optical data and the X-ray data derived from Swift/XRT, Swift/BAT and INTEGRAL IBIS/ISGRI monitoring. Analyzing the simultaneous optical and X-ray data, we found large-amplitude optical variations with regular patterns (the period: 30 min–2.5 hours, the amplitude: about 2 mag) at low luminosity (even at one hundredth of the Eddington luminosity) in the June outburst for the first time in black-hole X-ray transients. Prior to the outburst, repetitive variations in these objects had been observed only at high luminosity (e.g., GRS 1915+105) and the existing theories were developing to explain these observations before the outburst. The correlation between the repetitive optical variations and the X-ray ones was good. With the time lag between them and the multiwavelengths SED analysis, we demonstrated that the repetitive optical variability was mainly dominated by reprocessing of X-ray irradiation in the disk (Kimura et al. 2016). Also in the December outburst, the repetitive optical variations were detected in one term. In addition, we found that the X-ray emission delayed to the optical one by about 30–50 s. The time lag inconsistent with both disk reprocessing and synchrotron emission related to jet ejections. There is a possibility that many kinds of optical variations having different origins were observed during the two outbursts in 2015 in V404 Cyg.