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arXiv · 2001.07503

Rapid spectral transition of the black hole binary V404 Cyg

J. J. E. Kajava·C. Sánchez-Fernández·J. Alfonso-Garzón·S. E. Motta·A. Veledina
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Abstract

During the June 2015 outburst of the black hole binary V404 Cyg, rapid changes in the X-ray brightness and spectra were common. The INTEGRAL monitoring campaign detected spectacular Eddington-limited X-ray flares, but also rapid variations at much lower flux levels. On 2015 June 21 at 20 h 50 min, the 3-10 keV JEM-X data as well as simultaneous optical data started to display a gradual brightening from one of these low-flux states. This was followed 15 min later by an order-of-magnitude increase of flux in the 20-40 keV IBIS/ISGRI light curve in just 15 s. The best-fitting model for both the pre- and post-transition spectra required a Compton-thick partially covering absorber. The absorber parameters remained constant, but the spectral slope varied significantly during the event, with the photon index decreasing from $\Gamma \approx 3.7$ to $\Gamma \approx 2.3$. We propose that the rapid 20-40 keV flux increase was either caused by a spectral state transition that was hidden from our direct view, or that there was a sudden reduction in the amount of Compton down-scattering of the primary X-ray emission in the disk outflow.

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J. J. E. Kajava, C. Sánchez-Fernández, J. Alfonso-Garzón, S. E. Motta, A. Veledina. 2020-01-21. Rapid spectral transition of the black hole binary V404 Cyg. https://doi.org/10.1051/0004-6361/201937191

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