Iron line profiles in strong gravity

We describe a new code which can accurately calculate the relativistic effects which distort the emission from an accretion disc around a black hole. We compare our results for a disc which extends from the innermost stable orbit to 20rg in both Schwarzschild and maximal (a= 0.998) Kerr space–times with the two line profile codes which are on general release in the xspec spectral fitting package. These models generally give a very good description of the relativistic smearing of the line for this range of radii. However, these models have some limitations. In particular we show that the assumed form of the angular emissivity law (limb darkening or brightening) can make significant changes to the derived line profile where light bending is important. This is always the case for extreme Kerr space–times or high inclination systems, where the observed line is produced from a very large range of different emitted angles. In these situations the assumed angular emissivity can affect the derived radial emissivity. The line profile is not simply determined by the well defined (but numerically difficult) physical effects of strong gravity, but is also dependent on the poorly known astrophysics of the disc emission.