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Title: Rapid spectral transition of the black hole binary V404 Cygni
Authors: Kajava, J. J. E.
Sánchez Fernández, C.
Alfonso Garzón, J.
Motta, Sara E.
Veledina, A.
Keywords: Accretion;Accretion disks;Black hole physics;X rays: binaries;X rays: individuals: V404 Cyg
Issue Date: 13-Feb-2020
Publisher: EDP Sciences
DOI: 10.1051/0004-6361/201937191
Published version:
Citation: Astronomy and Astrophysics 634: A94(2020)
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 Γ ≈ 3.7 to Γ ≈ 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.
E-ISSN: 1432-0746
ISSN: 0004-6361
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