Predictions for Fourier-resolved X-ray spectroscopy from the model of magnetic flare avalanches above an accretion disc with hot ionized skin

The magnetic flare avalanches model of Poutanen & Fabian for X-ray variability of accreting black holes is combined with computations of the vertical structure of illuminated accretion discs in hydrostatic equilibrium. The latter predict the existence of a hot ionized skin, caused by the thermal instability of X-ray-illuminated plasma. The presence of such an ionized skin, with properties dependent on the disc radius, introduces a dependence of the emitted X-ray spectrum on the position on the disc. If the position is related to the time-scale of the flares, the X-ray energy spectra (both the primary continuum and the reprocessed component) gain an additional dependence on the Fourier frequency, beside that resulting from spectral evolution during a flare. We compute the Fourier frequency-resolved spectra in this model and demonstrate that the presence of the hot skin introduces trends opposite to those observed in black hole binaries. Furthermore, the flare profile is strongly constrained, if the Fourier frequency dependence arising from spectral evolution is to agree with observations.