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

¹ Centro de Astrobiología – Departamento de Astrofísica (CSIC-INTA), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
e-mail: julia@cab.inta-csic.es
² European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
³ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁴ Department of Astronomy, Graduate School of Science, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, 606-8502 Kyoto, Japan
⁵ Laboratoire AIM (CEA/IRFU – CNRS/INSU – Université Paris Diderot), CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
⁶ DTU Space – National Space Institute, Technical University of Denmark, Elektrovej, 327-328, 2800 Lyngby, Denmark

Received: 17 August 2018
Accepted: 7 October 2018

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.