Examinando por Autor "Veledina, A."

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Rapid spectral transition of the black hole binary V404 Cygni
(EDP Sciences, 2020-02-13) Kajava, J. J. E.; Sánchez Fernández, C.; Alfonso Garzón, J.; Motta, Sara E.; Veledina, A.; Science and Technology Facilities Council (STFC); Academy of Finland (AKA); Agencia Estatal de Investigación (AEI); Ministry of Education and Science of the Russian Federation (Minobrnauka); Kajava, J. J. E. [0000-0002-3010-8333]; Alfonso Garzón, J. [0000-0003-0852-3474]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
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.