© ESO 2020Kajava, J. J. E.Sánchez Fernández, C.Alfonso Garzón, J.Motta, Sara E.Veledina, A.Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-07372021-04-122021-04-122020-02-13Astronomy and Astrophysics 634: A94(2020)0004-6361https://www.aanda.org/articles/aa/full_html/2020/02/aa37191-19/aa37191-19.htmlhttp://hdl.handle.net/20.500.12666/259During 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.engAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttps://creativecommons.org/licenses/by-nc-nd/4.0/AccretionAccretion disksBlack hole physicsX rays: binariesX rays: individuals: V404 Cyginfo:eu-repo/semantics/article10.1051/0004-6361/2019371911432-0746info:eu-repo/semantics/openAccess