A global spectral study of black hole X-ray binaries

Abstract

We report on a consistent and comprehensive spectral analysis of the X‐ray emission of 25 black hole X‐ray binaries. All publicly available observations of the black hole binaries in the RXTE archive were analysed. Three different types of model were fitted to investigate the spectral changes occurring during an outburst. For the population, as well as for each binary and each outburst from each binary, we construct two diagnostic diagrams. The hardness intensity/luminosity diagram (HID/HLD), the X‐ray colour against the flux/luminosity of the binary, is most useful when studying a single binary. However, to compare different binary systems, the disc fraction luminosity diagram (DFLD) is more useful. The DFLD uses the luminosities of the disc and power‐law components to calculate the ratio of the disc luminosity to the total luminosity, resulting in a more physical value, which is analogous to the X‐ray colour calculated for X‐ray binaries. The tracks of the outbursts populate the DFLD more evenly than the HLD. We discuss the limitations of both diagnostic diagrams for the study of the X‐ray binary outbursts and clearly illustrate how the two diagrams map on to each other for real outburst data. The similarity of the X‐ray colour and disc fraction behaviour over time during an outburst originally seen in GX 339−4 data is seen in other sources' outbursts. We extract the peak luminosities in a single outburst, as well as the luminosities at the transitions away from and returning to the power‐law‐dominated state for each outburst. The distribution of the luminosities at the transition from the power‐law to the disc‐dominated state peaks at around 0.3LEdd, the same as the peak of the distribution of the peak luminosities in an outburst. Using the disc fraction to calculate the transition luminosities shows that the distributions of the luminosities for the transitions away from and returning to the power‐law‐dominated state are both broad and appear to overlap. Using the change in disc fraction to calculate the date when a transition occurred is not drastically different from obtaining the dates from changes in the timing behaviour of the X‐ray binary. In addition, we calculate the rate of motion of an X‐ray binary through the DFLD during an outburst, a diagnostic which has the potential to be used as a comparison with populations of active galactic nuclei. The fastest rate of motion is on the egress and ingress from the power‐law‐dominated state. A further region of increased speed through the diagram occurs in the disc‐dominated state on the return to the power‐law‐dominated state. Finally, we compare the measured X‐ray luminosities with a small number of contemporaneous radio measurements. Overall, this is the most comprehensive and uniform global study of black hole X‐ray binaries to date.

The radio/X-ray domain of black hole X-ray binaries at the lowest radio luminosities

“…Adopting a Bayesian regression model (Kelly 2007), and assuming errors of 0.3 dex in both r and X, the best-fitting slopes are: β = 0.62 ± 0.04 for GX339-4 (consistent with the value 0.62 ± 0.01 reported by Corbel et al 2013); β = 0.52 ± 0.07 for V404 Cyg (consistent with the value 0.51 ± 0.06 reported by Corbel, Körding & Kaaret 2008), and β = 0.72 ± 0.09 for XTE J1118+480 (this work; largely based on 2005 data from Brocksopp et al 2010 andDunn et al 2010). Employing a variety of clustering analysis algorithms in order to establish whether a single or multiple cluster model better describes the full data set (24 systems) yields inconclusive results; while two clusters are readily identified by various algorithms, different algorithms identify somewhat different memberships.…”

“…For XTE J1118+480 (shown as filled blue circles in Figure 2), we combine hard state observations from the 2000 outburst (data from Hynes et al 2000 and; these were already included in Gallo, Fender & Pooley 2003 and GMF12), the new VLA/Chandra observations discussed above, plus new data covering the 2005 outburst as reported by Brocksopp et al (2010) and Dunn et al (2010; for clarity, the 2005 outburst data are encircled by blue open diamonds in Figure 2), as follows. The observed 4.7 GHz radio flux densities reported by Brocksopp et al are converted to radio luminosities at 5 GHz (the standard frequency adopted for the compilation of sources) assuming a flat spectrum at all frequencies below 5 GHz, and adopting a distance of 1.7 kpc.…”

Black hole spin of Cygnus X-1 determined from the softest state ever observed

“…This is very different to the transient systems, where the mass accretion rate rises dramatically from quiescence to outburst, and the state transition takes place against a background of a strongly increasing mass accretion rate (e.g. the compilation of Dunn et al 2010). This may be the origin of the differences seen in the luminosity of the hard-to-soft spectral state change between Cyg X-1 and the transients.…”

“…The transients show large scale hysteresis, where the hard-to-soft transition during the rapid outburst rise can occur at a much higher (up to at least a factor of 10) luminosity than the reverse transition on the decline, and where the luminosity of the hardto-soft transition itself is variable between different outbursts of the same source (e.g. Smith et al 2002;Maccarone & Coppi 2003;Gierliński & Newton 2006;Gladstone et al 2007;Yu & Yan 2009;Dunn et al 2010). By contrast, in Cyg X-1, the hysteresis is only small scale, less than a factor of 2-3, and the hard-to-soft transition luminosity is fairly stable (Smith et al 2002;Zdziarski et al 2002;Meyer et al 2007).…”

The photon-index–time-lag correlation in black hole X-ray binaries

“…A narrow Gaussian component (line width σ < ∼ 0.9 keV) was added to account for the iron emission line at around 6.4 keV. The hydrogen column density was fixed to the values provided by Dunn et al (2010). For Cyg X-1 we used the value given by Hanke et al (2009).…”