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--20 $r_g$ 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 {\em angular} emissivity law (limb darkening or brightening) can make significant changes to the derived line profile. Lightbending effects are {\em never} negligible at these radii, so the observed line is produced from a range of different emitted angles, and this can affect the derived {\em 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.