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Hard X-ray behavior of V404 Cyg observed with INTEGRAL/IBIS

ATel #7759; L. Natalucci (IAPS/INAF, Italy), M. Fiocchi (IAPS/INAF, Italy), A. Bazzano (IAPS/INAF, Italy), P. Ubertini (IAPS/INAF, Italy), Erik Kuulkers (ESA/ESAC, Spain)
on 3 Jul 2015; 13:12 UT
Credential Certification: mariateresa fiocchi (mariateresa.fiocchi@iaps.inaf.it)

Subjects: X-ray, Gamma Ray, Binary, Black Hole, Transient

We analyzed public data obtained by IBIS during revolution 1554, starting at 2015-06-17 21:30 UTC while the source was extremely active (see ATel#7662). The light curves spanning 50 hours are characterized by intense flaring up to several tens of Crab. The brightest flare occurred at 2015-06-19 13:28:06.816 UTC where the source intensity reached ~6950 c/s in the full IBIS/ISGRI band (25-1000 keV), corresponding to a peak luminosity of the outburst of 4.3x10^38 erg s-1 assuming a distance of 2.4kpc. The evolution of the hardness ratio throughout the whole period is characterized by dramatic variations on time scales of tens of seconds or less. Spectral variations can be detected with high statistical significance up to ~200 keV by integrating spectra at the ~minute time scale. A preliminary analysis shows that the global spectrum is rather hard even at the highest luminosities (Gamma~1.5 in the 25-50 keV band) and is essentially characterized by two varying broad components. We find evidence of a Comptonized spectrum by thermal electrons at kTe~10 keV plus an additional high energy tail extending up to several hundred keV (see also ATel #7693).