Accretion flow diagnostics with X-ray spectral timing: the hard state of SWIFT J1753.5-0127

Abstract

Recent XMM-Newton studies of X-ray variability in the hard states of black hole X-ray binaries (BHXRBs) indicate that the variability is generated in the 'standard' optically thick accretion disc that is responsible for the multi-colour blackbody emission. The variability originates in the disc as mass-accretion fluctuations and propagates through the disc to 'light up' inner disc regions, eventually modulating the power-law emission that is produced relatively centrally. Both the covariance spectra and time-lags that cover the soft bands strongly support this scenario.Here, we present a comparative spectral-timing study of XMM-Newton data from the BHXRB SWIFT J1753.5−0127 in a bright 2009 hard state with that from the significantly fainter 2006 hard state to show for the first time the change in disc spectral-timing properties associated with a global increase in both the accretion rate and the relative contribution of the disc emission to the bolometric luminosity.We show that, although there is strong evidence for intrinsic disc variability in the more luminous hard state, the disc variability amplitude is suppressed relative to that of the power-law emission, which contrasts with the behaviour at lower luminosities where the disc variability is slightly enhanced when compared with the power-law variations. Furthermore, in the higher luminosity data the disc variability below 0.6 keV becomes incoherent with the power-law and higher energy disc emission at frequencies below 0.5 Hz, in contrast with the coherent variations seen in the 2006 data. We explain these differences and the associated complex lags in the 2009 data in terms of the fluctuating disc model, where the increase in accretion rate seen in 2009 leads to more pronounced and extended disc emission. If the variable signals are generated at small radii in the disc, the variability of disc emission can be naturally suppressed by the fraction of unmodulated disc emission arising from larger radii. Furthermore, the drop in coherence can be produced by disc accretion fluctuations arising at larger radii which are viscously damped and hence unable to propagate to the inner, power-law emitting region.

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

“…1999a, Nowak 2000 and other BHXRBs (e.g. Cassatella et al 2012, Grinberg et al 2014 we conclude that three variability components are sufficient to obtain a good description of the hard-state broad-band noise of GX 339-4 within the analysed frequency window (i.e. up to frequencies of 20 Hz).…”

“…All the PSDs show a similar shape, with two broad humps, whose intensity and characteristic frequencies clearly change as a function of both luminosity and energy. Grinberg et al 2014;Cassatella et al 2012). At frequencies lower than those covered here the variability power per unit logarithmic interval (νP ν ) is expected to drop in the hard state (band-limited noise).…”

X-ray reverberation around accreting black holes

“…3.2), and subsequently used in a number of other analyses of data from AGN and XRBs Middleton et al 2011;Cassatella et al 2012a;Kara et al 2013b;Cackett et al 2013). The covariance spectrum 6 is the cross-spectral counterpart of the rmsspectrum, which measures the rms amplitude of variability as a detailed function of energy.…”

Multi-Wavelength Variability

“…Since disc variations drive the harder X-ray variability (at least the lower-frequency component, , it seems likely that the stabilisation of disc variability is not strictly linked to the state transition itself but occurs continuously with the overall spectral softening of the source which begins in the hard state, perhaps linked to rises in overall accretion rate that trigger the state transition. Recent work looking at disc variabil-ity in XMM-Newton data from hard states of SWIFT J1753.5-0127 strongly supports this picture, showing that the disc is less variable in a softer, higher-luminosity hard state than in a harder, lower-luminosity harder state (Cassatella et al 2012a). Data from the Swift observatory, which enabled monitoring of the soft and hard X-ray variability of SWIFT J1753.5-0127 throughout a mini-outburst, also supports this picture (Kalamkar et al 2013).…”

“…The simplest interpretation is that this variability is intrinsic to the corona, hot inner flow, or base of the jet -whichever region produces the power-law emission. However, this need not be the case: mass accretion variations at higher Fourier frequencies in the disc could be filtered by the more extended disc emissivity profile to suppress the variability of the direct disc emission at these frequencies Cassatella et al 2012a). It remains an important open question as to whether or not the disc is responsible for driving all the non-jet continuum variability, or only that on longer time-scales.…”