A challenging view of the 2015 summer V404 Cyg outburst at high energy with INTEGRAL/SPI: The finale

Abstract

During its strong outburst of 2015 June/July, the X-ray transient V404 Cygni (= GS2023+338) was observed up to a level of 50 Crab in the hard X-ray domain. We focus here on a particularly intense episode preceeding a definitive decline of the source activity. We benefit from large signal-to-noise ratios to investigate the source spectral variability on a timescale of 5 minutes. A Hardness-Intensity study of three broad bands reveals clearly different behaviors at low and high energy (below and above around 100 keV). In particular, on two occasions, the source intensity varies by a factor of 3-4 in amplitude while keeping the same spectral shape. On the other hand, at the end of the major flare, the emission presents a clear anticorrelation between flux and hardness. These behaviors strongly suggest the presence of two spectral components related to emission processes varying in a largely independent way. The first component (E < 100-150 keV) is classically identified with a Comptonizing thermal electron population and requires either an unusual seed photon population or a specific geometry with strong absorbing/reflecting material. The second component is modeled by a cutoff power-law, which could correspond to a second hotter Comptonizing population or another mechanism (synchrotron, non-thermal Comptonization...). In the framework of such a model, Hardness-Intensity and Flux-Flux diagrams clearly demonstrate that the source evolution follows a well organized underlying scheme. They reveal unique information about the Hard X-ray emission processes and connections between them.

10 pages, 6 figures. Accepted for publication in ApJ

References in corpus (1)

• The Quiescent X-ray Spectrum of Accreting Black Holes

Cited by in corpus (9)

• The Positron Puzzle
• Background modelling for $\gamma$-ray spectroscopy with INTEGRAL/SPI
• The INTEGRAL view on Black Hole X-ray Binaries
• Black holes: Timing and spectral properties and evolution
• Optical and X-ray Correlations During the 2015 Outburst of the Black Hole V404 Cyg
• Rapid Optical Variations Correlated with X-rays in the 2015 Second Outburst of V404 Cygni (GS 2023$+$338)
• Rapid spectral transition of the black hole binary V404 Cyg
• Timing Analysis of V404 Cyg during Its Brightest Outburst with INTEGRAL/SPI
• MAXI J1535-571 2017 outburst Seen by INTEGRAL/SPI and Investigating the Origin of Its Hard Tail