X-ray spectra of accretion discs with dynamic coronae

We compute the X-ray spectra produced by non-static coronae atop accretion discs around black holes and neutron stars. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma, β = v / c . We show that an outflowing corona reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflection R and the X-ray spectrum slope Γ since both strongly depend on β . A similar correlation was observed and its shape was well fitted by the dynamic model. The scattering of soft radiation in an outflowing corona can also account for the observed optical–UV polarization pattern in active galactic nuclei.