http://arxiv.org/abs/1608.08802

Aims. Hard X-ray spectra of black hole binaries (BHB) are produced by Comptonization of soft seed photons by hot electrons near the black hole. The slope of the resulting energy spectra is governed by two main parameters: the electron temperature and the optical depth of the emitting plasma. Given the extreme brightness of V404 Cyg during the 2015 outburst, we aim to constraint the source spectral properties using an unprecedented time resolution in hard X-rays, and to monitor their evolution over the whole outburst. Methods. We have extracted and analyzed 602 X-ray spectra of V404 Cyg obtained during the 2015 June outburst by the IBIS/ISGRI instrument onboard INTEGRAL. We have performed time-resolved spectral analysis, using a time resolution ranging between 64 and 176000 seconds. The spectra were fitted in the 20-200 keV energy range. Results. We find that while the V404 Cyg light curve and soft X-ray spectra are remarkably different from other BHBs light curves, the spectral evolution of V404 Cyg in hard X-rays and the relations between the spectral parameters are consistent with those observed in other BHBs.We identify a hard branch where the electron temperature is anti-correlated with the hard X-ray flux, and a soft flaring branch where the relation reverses. In addition, we find that during the long X-ray plateaus detected at intermediate fluxes, the thermal Comptonization models fail to describe the spectra. Conclusions. We conclude that the hard branch in V404 Cyg is analogous to the canonical hard state of BHBs. V404 Cyg never seems to enter the canonical soft state, although the soft flaring branch bears resemblance to the BHB intermediate state and very high state. The X-ray plateaus are likely produced by absorption in a Compton-thick outflow, where the extreme absorption columns reduce the observed flux by a factor of about 10. [abridged]

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C. Sanchez-Fernandez, J. Kajava, S. Motta, et. al.
Thu, 1 Sep 16
8/74

Comments: Submitted to A&A