Rapid optical variations correlated with X-rays in the 2015 second outburst of V404 Cygni (GS 2023+338)

Abstract

We present optical multi-colour photometry of V404 Cyg during the outburst from December, 2015 to January, 2016 together with the simultaneous X-ray data. This outburst occurred less than 6 months after the previous outburst in June-July, 2015. 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 hours) optical variations even at low luminosity (0.01-0.1L Edd ). We found correlated optical and X-ray variations in two ∼1 hour 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 luminosity was L opt ∝ L 0.25−0.29 X 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 propagation of mass accretion flow to the inner optically-thin region with a speed comparable to the free-fall velocity.

Optical and X-ray correlations during the 2015 outburst of the black hole V404 Cyg

“…In every case except the MDM lightcurve from June 21 we see CCFs dominated by a single clear peak slightly after zero lag. These are all relatively broad, although vary substantially in width, and peak at lags of 10-50 s. This overlaps the 22.5 s and 34.8 s lags reported in the 2015 December outburst (Kimura et al 2017(Kimura et al , 2018. On June 21, the very large X-ray flare which is relatively uncorrelated with optical behaviour on long timescales dominates the MDM CCF producing a general rise towards late times.…”

“…8 to avoid compressing the outburst data, but we do find the extrapolation falls within about a factor of two of the quiescent accretion light. Kimura et al (2017) examined X-ray vs. optical correlations during the 2015 December outburst in a similar way, and found power-law indices of 0.25-0.29. These are based on much more limited time coverage than we have used, however, and sample a smaller dynamic range of luminosity.…”

The INTEGRAL view on Black Hole X-ray Binaries

“…The INTEGRAL and Swift spectra were compatible with a Comptonised continuum modified by a Compton-thick partial covering absorber [207]. Optical photometry [220] and spectroscopy, as well as monitoring in radio [334] also showed a behaviour comparable to what was observed in June, including similar, though less pronounced, P-Cyg optical lines and radio variability.…”

“…This lag was interpreted as the delay between fluctuations generated in the hot corona around the BH, that propagate up to the relativistic jet base about 1000 R g away, which is responsible for part of the optical emission [e.g., 266]. The correlated 1-minute time scale optical lags correspond to the light travel time delays from the BH to the outer disc, were instead likely produced by the re-processing of the X-rays in the outer accretion disc, or by the illuminated binary companion [220,1,186].…”

The 1989 and 2015 outbursts of V404 Cygni: a global study of wind-related optical features

“…The most remarkable example is found on day 8, when observations deviate from the trend, having a higher optical emission than those observed in days 1-3 at similar X-ray fluxes. Studies on the X-ray to optical correlation during the 2015J outburst of V404 have revealed a non-trivial relationship, showing from null to 30 minutes lags when comparing flares in different epochs (e.g., Rodriguez et al 2015, Kimura et al 2017; see also Alfonso-Garzon et al 2018 in prep.). These lags may be responsible for the deviation from the trend in our lower time-resolution observations.…”