Concave accretion discs and X-ray reprocessing

Spectra of Seyfert Is are commonly modelled as emission from an X-ray illuminated flat accretion disc orbiting a central black hole. This provides both a reprocessed and direct component of the X-ray emission as required by observations of individual objects and possibly of the cosmological X-ray background. However, there is observational motivation to consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction $> 1$ in individual Seyferts and the X-ray background, and (2) the commonality of a sharp iron line peak for Seyferts at 6.4KeV despite a strong dependence of peak location on inclination angle for flat disc models. Here it is shown that a concave disc may not only provide a larger total fraction of reprocessed photons but can also reprocess a much larger fraction of photons in its outer regions when compared to a flat disc. This reduces the sensitivity of the 6.4KeV peak location to the inner disc inclination angle because the outer regions are less affected by Doppler and gravitational effects.