Rapid spectral transition of the black hole binary V404 Cyg

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.

6 pages, 2 figures, accepted to A&A

References in corpus (11)

• Modelling the behaviour of accretion flows in X-ray binaries
• A rapidly-changing jet orientation in the stellar-mass black hole V404 Cygni
• Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion
• Spectral variability in Cygnus X-3
• Investigating state transition luminosities of Galactic black hole transients in the outburst decay
• Time-dependent models of two-phase accretion discs around black holes
• Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni
• Accretion and Outflow in V404 Cyg
• Multi-Wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546
• Suzaku Observation of the Black Hole Binary 4U 1630--47 in the Very High State
• A challenging view of the 2015 summer V404 Cyg outburst at high energy with INTEGRAL/SPI: The finale

Cited by in corpus (3)

• Short Timescale Evolution of the Polarized Radio Jet during V404 Cygni's 2015 Outburst
• The Black Hole Candidate Swift J1728.9$-$3613 and the Supernova Remnant G351.9$-$0.9
• Rapid onset of a Comptonisation zone in the repeating tidal disruption event XMMSL2 J140446.9-251135