A Physical Model for the Spectral-Timing Properties of Accreting Black Holes

Ra'ad D. Mahmoud, Chris Done

Published 2018-03-13Version 1

The rapid X-ray variability of black hole accretion flows in the low/hard state has a strong energy dependence, even looking only above 3 keV where the emission is dominated by Comptonisation. This is clear from the different shapes of the power spectra at different energies, and in the complex pattern of the time lags between distinct energy bands. Our analytic technique fits the energy-dependent power spectra and time lags using a physical model based on fluctuations propagating through a spectrally inhomogeneous flow. The spectral components are set by jointly fitting to the time averaged and Fourier resolved spectra. This framework simultaneously approximates the energy spectra, power spectra in different energy bands and time lags between bands from high quality Cygnus X-1 data where clear structure in the timing statistics is present. We find that the model features demanded by the data are (1) enhanced variability and emission at key radii within the flow, (2) destruction of propagating fluctuations near positions of enhanced turbulence/emission, and (3), stratification of the Comptonising flow into at least three regions, each producing a distinct Compton component. These results establish the importance of specific radii in the Comptonising zone, likely associated with the disc truncation, the inner edge of the flow, and/or the jet launch radius.