Concave accretion discs and X‐ray reprocessing

Spectra of Seyfert 1s are commonly modelled as emission from an X‐ray‐illuminated flat accretion disc orbiting a central black hole. This provides both reprocessed and direct components of the X‐ray emission, as required by observations of individual objects, and possibly a fraction of the cosmological X‐ray background. There is some observational motivation for us to at least consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction ≳1/2 in some Seyferts and possibly in the X‐ray background, and (2) the commonality of a sharp iron 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 regions compared with a flat disc. This reduces the sensitivity of the 6.4‐keV peak 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 concavity. If the X‐ray source is anisotropic, the location of iron line peak can still be determined by concavity but the total reflected fraction need not be as large as for the isotropic emitter case. The geometric calculations herein are applicable to general accretion disc systems illuminated from the centre.