The soft component and the iron line as signatures of the disc inner radius in Galactic black hole binaries

Abstract

The inner radius of the accretion disc around a black hole in the low/hard state can be measured in one of two ways. Firstly, via the extent of broadening of the iron emission line, and secondly, from the luminosity and temperature of the weak soft component seen in this state, assuming it is the disc. We use both of these methods on all the low/hard state spectra taken in timing mode of XMM-Newton 's EPIC-pn. We find that the two methods are not consistent with each other, and the difference is not always in a single direction. The two methods are neither model independent, nor are they independent of current calibration issues. We find that the remaining small residuals in the EPIC-pn timing mode response at the ≤ 3% level can have a dramatic effect on the fit parameters for the reflected spectrum. There is also a mismatch in cross-calibration with RXTE, which makes it difficult to use simultaneous data to extend the bandpass of the spectral fits. Nonetheless, it is clear from the data that the iron line is noticeably broader and stronger at higher L/L Edd , which is consistent with the truncated disc models. We also show that it is likely that the soft component changes character, from a stable component consistent with a truncated disc at high L/L Edd , to a variable one with much smaller radius at low L/L Edd . This adds to growing evidence for a complex soft component in the low/hard state, possibly resulting from clumps torn from the edge of the truncated disc.

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

“…Previous analyses of the same (or part of the) data sets analysed in this paper (Wilkinson & Uttley 2009;Kolehmainen et al 2014) revealed the presence of a significant contribution from the accretion disc-thermal emission in the soft band. Thus we used the XMM-Newton data to measure the disc-thermal emission-to-total flux ratio in the soft band and verify whether the drop of fractional rms is related to spectral variations in this band.…”

“…tbabs * [diskbb+nthcomp], with the Comptonization high energy cut-off fixed at 100 keV and the seed photon temperature tied to the disc temperature), and excluding the range of energies 4 − 7 keV, dominated by the Fe Kα line. We also excluded the energies around the edges of the response matrix, since residuals at these energies might be an artifact of uncorrected XRL effects and/or charge transfer inefficiency (Kolehmainen et al 2014). The contribution from the disc emission strongly depends on the cold absorption column density parameter, N H .…”

“…The way these variability properties extrapolate to soft X-ray energies (< 3 keV) during the hard state is currently poorly known. Since, during the hard state the disc component gives nonnegligible contribution in the soft X-ray energy band ( < ∼ 1 keV; Tomsick et al 2008;Kolehmainen, Done, & Díaz-Trigo 2014), to some extent we would expect to observe behaviours similar to those characterizing the disc during the soft state. However, the recent use of data from the EPIC-pn detector onboard XMM-Newton (sensitive down to ∼0.3 keV), for the spectral-timing analysis of BHXRBs, unveiled the existence of high levels of intrinsic (on time scales of > ∼ 1 s) disc variability in the hard state (Wilkinson & Uttley, 2009).…”

Spectral analysis of theXMM–Newtondata of GX 339–4 in the low/hard state: disc truncation and reflection

“…Hence, one should avoid using detectors which are severely affected by this effect (Done & Díaz Trigo 2010). Kolehmainen, Done & Díaz Trigo (2014) (hereafter KDD14) calculated the inner radius both from the modelling of the disc and that of the reflected component using the data taken by the XMM-Newton EPIC-pn detector in timing mode. Though their study indicates highly truncated disc, the radii calculated by the two methods do not agree with each other.…”

SPECTRAL AND TIMING PROPERTIES OF THE BLACK HOLE X-RAY BINARY H1743–322 IN THE LOW/HARD STATE STUDIED WITHSUZAKU

“…This spectral shape is generally described as unsaturated Compton scattering of the soft X-ray photons from the accretion disk by thermal electrons in hot inner flow or corona. Previous studies suggested that the standard disk is truncated in the low/hard state and eventually extends inward according to the increase of X-ray luminosity (Tomsick et al 2009;Shidatsu et al 2011b;Yamada et al 2013;Kolehmainen et al 2014). However, in what luminosity the standard disk reaches the ISCO is still controversial.…”