Optical/X-ray correlations during the V404 Cygni June 2015 outburst

Abstract

Context. We present a multiwavelength analysis of the simultaneous optical and X-ray light curves of the microquasar V404 Cyg during the June 2015 outburst. Aims. We have performed a comprehensive analysis of all the INTEGRAL/IBIS, JEM–X, and OMC observations during the brightest epoch of the outburst, along with complementary NuSTAR, AAVSO, and VSNET data, to examine the timing relationship between the simultaneous optical and X-ray light curves, in order to understand the emission mechanisms and physical locations. Methods. We have identified all optical flares that have simultaneous X-ray observations, and performed a cross-correlation analysis to estimate the time delays between the optical and soft and hard X-ray emission. We also compared the evolution of the optical and X-ray emission with the hardness ratios. Results. We have identified several types of behaviour during the outburst. On many occasions, the optical flares occur simultaneously with X-ray flares, but at other times, positive and negative time delays between the optical and X-ray emission are measured. Conclusions. We conclude that the observed optical variability is driven by different physical mechanisms, including reprocessing of X-rays in the accretion disc and/or the companion star, interaction of the jet ejections with surrounding material or with previously ejected blobs, and synchrotron emission from the jet.

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

“…Light travel time delays from the disc should then range from 0-80 s, with the response weighted towards shorter lags within the range (O'Brien et al 2002). Similar estimates were made independently by Alfonso-Garzón et al (2018).…”

“…Observing lags longer than possible from geometrical light travel times alone is then not a problem for the reprocessing model, and these prolonged reprocessing times could contribute to the extended tails to smooth decays described in Section 3.2. In this respect we disagree from Alfonso-Garzón et al (2018) who only consider geometrical light travel time delays for reprocessing.…”

Swift UVOT observations of the 2015 outburst of V404 Cygni

“…With a black hole mass of 9 M , it has been estimated that the the separation between the black hole and the companion is ∼ 2.2×10 12 cm or ∼ 75 light seconds (Rodriguez et al 2015;Gandhi et al 2017). The short time lag observed for the segments of our optical and X-ray light curves, similar to that reported by Gandhi et al (2015), Kimura et al (2016) and Alfonso-Garzón et al (2018), is consistent with this picture. However, our typical measured optical lag of < 35 sec, may suggest that the u-band emission from reprocessing does not originate at the very outskirts of the disk, but at a radius of 1 × 10 12 cm.…”

Rapid spectral transition of the black hole binary V404 Cygni

“…1. The data show that the optical V-band and the soft X-ray brightness first begins to rise at around 20 h 54 min, while the hard X-ray flux remains constant (see Alfonso-Garzón et al 2018, for a more detailed crosscorrelation analysis). The V -band and soft X-ray brightness hit a local maximum at the same time, which is then followed by a leveling of the JEM-X count rate and an immediate and gradual decrease of the optical flux.…”

“…The JEM-X and ISGRI responses were logarithmically rebinned to obtain 23 and 28 channels, respectively. The OMC light curves were also extracted with OSA v.10.2, as detailed in Alfonso-Garzón et al (2018).…”