Hard X-ray variability of V404 Cygni during the 2015 outburst

Abstract

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 (T e ) and the optical depth (τ) of the emitting plasma. Given the extreme brightness of V404 Cyg during the 2015 outburst, we aim to constrain the source spectral properties using an unprecedented time resolution in hard X-rays, and to monitor the evolution of T e and τ over the outburst. Methods. We have extracted and analyzed 602 X-ray spectra of V404 Cyg obtained by the IBIS/ISGRI instrument on-board IN-TEGRAL during the 2015 June outburst, using effective integration times ranging between 8 and 176000 seconds. We have fit the resulting spectra in the 20-200 keV energy range. Results. We find that while the light curve and soft X-ray spectra of V404 Cyg are remarkably different from those of other BHBs, 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 T e is anti-correlated with the hard X-ray flux, and a soft flaring branch where the relation reverses. In addition, we find that during long X-ray plateaus detected at intermediate fluxes, the thermal Comptonization models fail to describe the spectra. However, the statistics improve if we allow N H to vary freely in the fits to these 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 ultraluminous state. The X-ray plateaus are likely the result of absorption by a Compton-thick outflow (N H 10 24 cm −2 ) which reduces the observed flux by a factor of about 10. Variable covering of the central source by this Compton-thick material may be the reason for the complicated light curve variability, rather than intrinsic source variability.

V404 Cyg: an INTEGRAL and Swift X-ray view of an obscured microquasar

“…We concentrate on the variations of the source spectral properties induced by the presence of a variable neutral absorber local to the source, and on the variability of the source hard X-ray spectrum, which shows slower and and likely accretion driven changes in the source. Results from the analysis of INTEGRAL /IBIS/ISGRI data were reported by [27]. Results based on the Swift/XRT observations were presented by Motta et al 2017c.…”

“…In both panels, the points are colour-coded according to the flux, with redder points corresponding to higher fluxes. Each point corresponds to a variable exposure stretch of data, whose time duration has been determined based on the source flux (see [27] and Motta et al 2017c for details). Note that the flux scale is different in the two panels due to the different energy bands of INTEGRAL /ISGRI and Swift/XRT.…”

“…We fitted the spectra in the 20-200 keV energy range using spectral models based on thermal Comptonization (NTHCOMP and COMPPS; see [27] for details), depending on the following spectral parameters: the spectral slope, Γ, electron temperature, T e , optical depth, τ, and column density, N H . We also measured the X-ray flux F x in the 20-200 keV energy band from every spectrum we fitted.…”

Soft gamma-ray emission from the V404 Cygni outburst observed with INTEGRAL/PICsIT

“…Many of those studies were based on the INTEGRAL's [13] observations carried out almost continuously for about a month. Thanks to the enormous brightness of the source it was possible to explore its spectral properties in the hard X-ray and soft γ-ray bands to time scales down to minutes [6,7,9].…”

“…Recently a set of 602 ISGRI spectra extracted for the whole INTEGRAL observing period was analysed with two Comptonization models [9]. Fitted kT e values formed a relatively narrow distribution centered around 40 keV, whereas the plasma optical depth values were found in a broad range between 0 and 6.…”