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

TL;DR

This study analyzes simultaneous optical and X-ray observations of V404 Cyg during its 2015 outburst, revealing diverse timing behaviors and multiple emission mechanisms such as reprocessing, jet interactions, and synchrotron emission.

Contribution

It provides a comprehensive multiwavelength analysis of V404 Cyg's outburst, identifying various optical-X-ray correlation patterns and physical emission processes.

Findings

Optical flares often occur simultaneously with X-ray flares.

Variable time delays indicate multiple emission mechanisms.

Optical variability driven by reprocessing, jet interactions, and synchrotron emission.

Abstract

We present a multiwavelength analysis of the simultaneous optical and X-ray light curves of the microquasar V404 Cyg during the June 2015 outburst. 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. We have identified all optical flares which have simultaneous X-ray observations, and performed cross-correlation analysis to estimate the time delays between the optical and soft and hard X-ray emission. We have also compared the evolution of the optical and X-ray emission with the hardness-ratios. 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. 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.