Disc-driven X-ray Variability And Reverberation In Hard State Black Hole X-ray Binaries

Details Disc-driven X-ray Variability And Reverberation In Hard State Black Hole X-ray Binaries Disc-driven X-ray Variability And Reverberation In Hard State Black Hole X-ray Binaries

Feb 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010head...11.1109u&link_type=abstract

American Astronomical Society, HEAD meeting #11, #11.09; Bulletin of the American Astronomical Society, Vol. 41, p.672

Astronomy and Astrophysics

Astronomy

Scientific paper

Black hole XRB X-ray variability contains information about the accretion and emission mechanisms in strong gravity, but to unlock this information and understand the very rich phenomenology of X-ray timing properties (band-limited noise, QPOs, low-frequency noise), we need to understand the origin of the variations. For many years it has been assumed that the Lorentzian components seen in Fourier power-spectra of the hard state BHXRBs are associated to characteristic time-scales in the accretion flow. However, the dominance of the hard power-law component in the SED, and the weakness of variability seen in the disk-dominant soft state, led to suggestions that this unstable part of the flow must be optically thin, e.g. an ADAF or coronal flow inside the inner-disk truncation radius, and is not the standard accretion disk which contributes only a fraction of the hard state luminosity. We use XMM-Newton EPIC-pn timing mode observations of hard state BHXRBs to show that the variability is in fact driven by the standard accretion disk, as shown by a sharp increase in the hard-soft time-lag which arises at energies where the disk starts to dominate the X-ray spectrum. The disk variations clearly lead the power-law variations, which are probably generated by fluctuations in accretion rate in the disk before propagating into the inner hot flow. On time-scales around a tenth of a second however, the pattern reverses and the soft photons start to lag harder photons with delays around 1 msec, i.e.the light-crossing time for a few tens of gravitational radii. This result suggests that X-ray 'reverberation' of the disk dominates the lags on the shortest time-scales, confirming the potential for current and future observatories to use X-ray reverberation to map the disk.
Funded by the EU under the Initial Training Network "The Black Hole Universe"

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