Correlated optical, X-ray, and γ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni^⋆

¹ Laboratoire AIM, UMR 7158, CEA/CNRS/Université Paris Diderot, CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
e-mail: jrodriguez@cea.fr
² Max Planck Computing and Data Facility, 85748 Garching, Germany
³ Centro de Astrobiología (CSIC-INTA), ESAC Campus, POB 78, 28691 Villanueva de la Cañada, Spain
⁴ Max Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
⁵ Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA
⁶ Laboratoire APC, UMR 7164, CEA/CNRS/Université Paris Diderot, 75013 Paris, France
⁷ Space Science Lab, University of California, Berkeley, CA 94720, USA
⁸ DTU Space – National Space Institute, Technical University of Denmark, Elektrovej 327-328, 2800 Lyngby, Denmark
⁹ Universitäts-Sternwarte Muenchen, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Muenchen, Germany
¹⁰ Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹¹ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
¹² New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
¹³ Dr. K. Remeis-Sternwarte & Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

Received: 23 July 2015
Accepted: 31 August 2015

Abstract

After 25 years of quiescence, the microquasar V404 Cyg entered a new period of activity in June 2015. This X-ray source is known to undergo extremely bright and variable outbursts seen at all wavelengths. It is therefore an object of prime interest to understand the accretion-ejection connections. These can, however, only be probed through simultaneous observations at several wavelengths. We made use of the INTEGRAL instruments to obtain long, almost uninterrupted observations from 2015 June 20, 15:50 UTC to June 25, 4:05 UTC, from the optical V band up to the soft γ-rays. V404 Cyg was extremely variable in all bands, with the detection of 18 flares with fluxes exceeding 6 Crab (20–40 keV) within three days. The flare recurrence can be as short as ~20 min from peak to peak. A model-independent analysis shows that the >6 Crab flares have a hard spectrum. A simple 10–400 keV spectral analysis of the off-flare and flare periods shows that the variation in intensity is likely to be only due to variations of a cut-off power-law component. The optical flares seem to be at least of two different types: one occurring in simultaneity with the X-ray flares, the other showing a delay greater than 10 min. The former could be associated with X-ray reprocessing by either an accretion disk or the companion star. We suggest that the latter are associated with plasma ejections that have also been seen in radio.