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 disk from 6-20r_g in both Schwarzchild and extreme Kerr spacetimes with the two line profile codes which are on general release in the XSPEC spectral fitting package. These are generally accurate at the 10-20% level for this range of radii, but have some drawbacks in terms of assumptions and/or resolution. 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. Lightbending effects are never negligible at these radii, so the observed line is produced from a range of different emitted angles, and this can affect the derived radial emissivity law. 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. The inference of direct tapping of the spin energy due to the derived steepness of the radial emissivity in MCG-6-30-15 of Wilms et al. (2001) is dependent on this hidden assumption, so may be premature