Accretion disc variability in the hard state of black hole X-ray binaries

By T. Wilkinson, P. Uttley. 2009

143 citations recorded by the source corpus.

XMM–Newton X-ray spectra of the hard state black hole X-ray binaries (BHXRBs) SWIFT J1753.5?0127 and GX 339?4 show evidence for accretion disc blackbody emission, in addition to hard power laws. The soft and hard band power spectral densities (PSDs) of these sources demonstrate variability over a wide range of time-scales. However, on time-scales of tens of seconds, corresponding to the putative low-frequency Lorentzian in the PSD, there is additional power in the soft band. To interpret this behaviour, we introduce a new spectral analysis technique, the 'covariance spectrum', to disentangle the contribution of the X-ray spectral components to variations on different time-scales. We use this technique to show that the disc blackbody component varies on all time-scales, but varies more, relative to the power law, on longer time-scales. This behaviour explains the additional long-term variability seen in the soft band. Comparison of the blackbody and iron line normalizations seen in the covariance spectra in GX 339?4 implies that the short-term blackbody variations are driven by thermal reprocessing of the power-law continuum absorbed by the disc. However, since the amplitude of variable reflection is the same on long and short time-scales, we rule out reprocessing as the cause of the enhanced disc variability on long time-scales. Therefore, we conclude that the long time-scale blackbody variations are caused by instabilities in the disc itself, in contrast to the stable discs seen in BHXRB soft states. Our results provide the first observational evidence that the low-frequency Lorentzian feature present in the PSD is produced by the accretion disc.

Subfields: Astronomy and Astrophysics, Biomedical Engineering

View Paper: https://www.semanticscholar.org/paper/4864f83a48df2cb74a1934dd3213f5397159e890

How researchers see this paper

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In Astronomy and Astrophysics

1. A. C. Fabian
2. C. Done
3. J. Miller
4. Ramesh Narayan
5. E. Cackett
6. Aya Kubota
7. E. Kara
8. A. Ingram

In Biomedical Engineering

1. C. Done
2. J. Miller
3. A. Ingram
4. A. C. Fabian
5. P. Casella
6. Aya Kubota
7. P. Uttley
8. Ramesh Narayan

Citations in the corpus

• A quasi-periodic modulation of the iron line centroid energy in the black hole binary H1743−322 (2016, methodology): “Since the light curves from each energy channel are well correlated, the covariance gives a good measure of the power spectrum with smaller statistical errors (Wilkinson & Uttley 2009).”
• Rapid optical and X-ray timing observations of GX 339-4: multi-component optical variability in the low/hard state (2010, background): “Such a changeover in fluctuations can naturally cause a loss of coherence (e.g. Vaughan & Nowak 1997), especially if the origin of the low and high frequency Lorentzians is independent, which is likely to be the case (e.g. Wilkinson & Uttley 2009).”
• A physical model for the continuum variability and QPO in accreting black holes (2011, background): “5-0127 (Wilkinson & Uttley 2009), suggesting that there is considerable complexity in the disc truncation (see also Chiang et al 2010).”
• X-ray reverberation around accreting black holes (2014, methodology): “The development of the covariance spectrum allowed the study of the detailed energy dependent variability of X-ray binaries in the low-hard state, which revealed that the emission of the accretion disc in that state was significantly variable (Wilkinson & Uttley 2009).”