Modelling the Energy Dependence of BHB Flows [HEAP]

We build a full spectral-timing model for the low/hard state of black hole binaries assuming that the spectrum of the X-ray hot flow can be described by two Comptonisation zones. Slow fluctuations stirred up at the largest radii/softest spectral region of the flow propagate down to modulate the faster fluctuations produced in the harder spectral region close to the black hole. The observed spectrum and variability are produced by summing over all regions in the flow, including its emission reflected from the truncated disc. This produces energy-dependent Fourier lags qualitatively similar to those seen in the data. We apply this model to archival RXTE data from Cyg X-1, using the time averaged energy spectrum together with an assumed emissivity to set the radii of the soft and hard Compton regions. This predicts the Fourier Power Density Spectra and lags for any energy bands when combined with a viscous frequency prescription. We find that the power spectra cannot be described by any smooth model of generating fluctuations, and instead require that there are specific radii in the flow where noise is preferentially produced. We also find that damping is required to prevent all the variability power stirred up at large radii being propagated into the inner regions. Even with these additions, we can fit either the power spectra at each energy, or the lags, but not both. We conclude that either the spectra are more complex than two zone models, or that other processes are important in forming the variability.

Read this paper on arXiv…

R. Mahmoud and C. Done
Mon, 19 Jun 17

Comments: 14 pages, 11 figures