Rapid Optical Variability in the 2015 Outbursts of V404 Cygni

V404 Cyg is one of black-hole X-ray transients, which are characterized by sporadic outbursts lasting for a few hundreds days (Tanaka & Shibazaki 1996). This object entered outbursts twice in 2015 after the 26-years quiescence, and we performed their optical photometry through the Variable Star Network (VSNET) team. We detected its optical short-term and large-amplitude variations for the first time in the 2015 June-July outburst. Some of them have repetitive patterns with amplitudes of 0.1-2.5 mag on timescales of 5-150 min, and we found these optical variations chased the simultaneous X-ray variations by comparing the simultaneous optical data and X-ray data which were taken by Swift/INTEGRAL satellites. Additionally, their optical flux were not explained only by Synchrotron emission expected by the radio flux. We thus concluded that the X-ray reprocessing was dominant in these variations (Kimura et al. 2016). Also in the 2015 December outburst, the rapid and high-amplitude optical variations were observed, and we confirmed the dominant X-ray reprocessing in these variations by the 30-s optical delay against the simultaneous X-ray variations (Kimura et al. 2017). Repetitive variability is also observed in a few other X-ray binaries. It was always observed when the luminosity was close to the Eddington luminosity, and hence, existing theory assumes the Eddington luminosity is a key condition. V404 Cyg sometimes exceeds the Eddington luminosity in the June outburst, but the repetitive patterns of optical variability in this object were detected even at low luminosity. In this presentation, we focus on the violent variability in this system and discuss its mechanism.