@article{c640f6415c76456ba0a23c3cc94e6dcb,
title = "Correlated optical, X-ray, and γ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni",
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.",
keywords = "Accretion, accretion disks, Radio continuum: stars, Stars: black holes, Stars: individual: V404 Cygni, X-rays: binaries",
author = "J. Rodriguez and {Cadolle Bel}, M. and J. Alfonso-Garz{\'o}n and T. Siegert and Zhang, {X. L.} and V. Grinberg and V. Savchenko and Tomsick, {J. A.} and J. Chenevez and M. Clavel and S. Corbel and R. Diehl and A. Domingo and C. Gouiff{\`e}s and J. Greiner and Krause, {M. G.H.} and P. Laurent and A. Loh and S. Markoff and Mas-Hesse, {J. M.} and Miller-Jones, {J. C.A.} and Russell, {D. M.} and J. Wilms",
note = "Funding Information: We warmly thank the referee for the useful report that helped us to improve the quality of this paper. We also thank the INTEGRAL teams and planners for their prompt reaction and the scheduling of these observations. J.R., M.C., S.C. acknowledge funding support from the French Research National Agency: CHAOS project ANR-12-BS05-0009 ( http://www.chaos-project.fr ), and from the UnivEarthS Labex program of Sorbonne Paris Cit{\'e} (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). XLZ acknowledges funding through DLR 50 OG 1101. M.G.H.K. was supported by the Deutsche Forschungsgemeinschaft under DFG project number PR 569/10-1 in the context of the Priority Program 1573 Physics of the Interstellar Medium. This work was supported by NASA through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 for the Chandra X-Ray Center and Science Instruments. R.D. and X.L.Z. acknowledge support through the Deutsches Zentrum f{\"u}r Luft- und Raumfahrt e.V. (DLR) 50 OG 1101. O.M.C. activities are supported by Spanish MINECO grant ESP2014-59789-P. This study is based on observations made with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states, Poland and with the participation of Russia and the USA.",
year = "2015",
month = sep,
day = "1",
doi = "10.1051/0004-6361/201527043",
language = "English (US)",
volume = "581",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
}