Concave accretion discs and X-ray reprocessing

Spectra of Seyfert Is are commonly modelled as emission from an X-ray-illum inated flat accretion disc orbiting a central black hole. This provides bot h reprocessed and direct components of the X-ray emission, as required by o bservations of individual objects, and possibly a fraction of the cosmologi cal X-ray background. There is some observational motivation for us to at l east consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction greater than or similar to 1/2 in some Seyferts and possibly in the X-ray background, and (2) the commonality of a sharp i ron line peak for Seyferts at 6.4 keV despite a 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 regio ns compared with a flat disc. This reduces the sensitivity of the 6.4-keV p eak location to the inner disc inclination angle because the outer regions are less affected by Doppler and gravitational effects. If the X-ray source is isotropic, the reprocessed fraction is directly determined by the conca vity. If the X-ray source is anisotropic, the location of iron line peak ca n still be determined by concavity but the total reflected fraction need no t be as large as for the isotropic emitter case. The geometric calculations calculations herein are applicable to general accretion disc systems illum inated from the centre.