Rapid optical variations correlated with X-rays in the 2015 second outburst of V404 Cygni (GS 2023+338)
Nogami, Daisaku; Shidatsu, Megumi; Kato, Taichi; Isogai, Keisuke; Kimura, Mariko; Kojiguchi, Naoto; Itoh, Hiroshi; Matsumoto, Katsura; Tordai, Tamás; Kiyota, Seiichiro; Dubovsky, Pavol A.; de Miguel, Enrique; Pavlenko, Elena P.; Pickard, Roger D.; Sosnovskij, Aleksei A.; Babina, Julia V.; Kasai, Kiyoshi; Stein, William L.; Goff, William; Sugiura, Yuki; Yamada, Eiji; Maeda, Yutaka; Miyashita, Atsushi; Cook, Lewis M.; Kudzej, Igor; Tak, Hyungsuk; Tatsumi, Taiki
Japan, United States, Hungary, Switzerland, United Kingdom, Slovakia, Spain, Ukraine
Abstract
We present optical multicolour photometry of V404 Cyg during the outburst from 2015 December to 2016 January together with the simultaneous X-ray data. This outburst occurred less than six months after the previous outburst in 2015 June-July. These two outbursts in 2015 were of a slow-rise and rapid-decay type and showed large-amplitude (∼2 mag) and short-term (∼10 min-3 h) optical variations even at low luminosity (0.01-0.1LEdd). We found correlated optical and X-ray variations in two ∼1 h time intervals and obtained a Bayesian estimate of an X-ray delay against the optical emission, which is ∼30-50 s, during those two intervals. In addition, the relationship between the optical and X-ray luminosities was L_opt ∝ L_X^{0.25-0.29} at that time. These features cannot be easily explained by the conventional picture of transient black hole binaries, such as canonical disc reprocessing and synchrotron emission related to a jet. We suggest that the disc was truncated during those intervals and that the X-ray delays represent the required time for the propagation of mass accretion flow to the inner optically thin region with a speed comparable to the free-fall velocity.