Statistics – Computation
Scientific paper
Apr 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..238..319a&link_type=abstract
Black Holes from Stars to Galaxies -- Across the Range of Masses. Edited by V. Karas and G. Matt. Proceedings of IAU Symposium #
Statistics
Computation
Scientific paper
We model, phenomenologically, fluctuations in the accretion flow to explain the variability properties observed in X-ray light curves of accreting black hole systems. We show how the information contained in the timing properties of these light curves can be used to constrain the geometry and behaviour of the accretion disc. In our computational model, which is based on existing analytical models in the literature, variability is produced by accretion rate fluctuations that propagate through the accretion flow, modulating the emission on a broad range of time-scales. The X-ray light curves are assumed to be emitted by a radially-extended region, which acts as a low-pass filter reproducing the high frequency break observed in the power spectral density (PSD) of Active Galactic Nuclei and black hole X-ray binaries. By assigning different radial emissivity profiles to the energy bands we obtain PSDs with higher break frequencies and/or flatter high frequency slopes for higher energy bands, as normally observed in the data. Different emissivity profiles also introduce time lags between the corresponding energy bands without requiring any spectral evolution of the emitting region. Keeping the characteristic frequencies and propagation speeds tied to the local viscous time-scale of a thick disc produces time lags of similar magnitude and Fourier frequency dependence as those observed in the data. The predicted lags are quite robust to changes in the emissivity profile, disc parameters and the effects of damping on the fluctuations as they propagate inwards, which may help to explain the similarity of the lag spectra in the low/hard and high/soft states of Cyg X-1. Finally, the light curves resulting from this common emitting region are highly spectrally coherent. Coherence losses at the level of a few percent appear for certain model configurations, increasing with energy separation of the bands and with Fourier frequency. We compare model predictions to X-ray data from the BHXRB Cyg X-1 and show that this general scheme can reproduce simultaneously the time lags, energy-dependence of the PSD and coherence. Coherence losses at the level of a few percent appear for certain model configurations, increasing with energy separation of the bands and with Fourier frequency. We compare model predictions to X-ray data from the Narrow Line Seyfert 1 galaxy ngc and the BHXRB Cyg X-1 and show that this general scheme can reproduce simultaneously the time lags, energy-dependence of the PSD and coherence [ADS note: Abstract appears to have been truncated here.]
Arevalo Patricia
McHardy Ian
Uttley Phil
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