The profile and equivalent width of the X-ray iron emission line from a disc around a Kerr black hole

Abstract

Recent X-ray observations have shown broad, skewed iron line emission from Seyfert 1 galaxies, which is explained by the emission being fluorescence on a disc close to a black hole. During one interval, the line in MCG-6-30-15 was so broad and redshifted that a Kerr black hole is implied. We are therefore studying the effects of the Kerr metric on the line profile, and extending the work by Laor and Kojima which dealt only with extreme values of the spin parameter. Here we report that the spin parameter of the black hole in MCG-6-30-15 is high (a/M > 0.94), and invert the line profile to obtain the disc emissivity profile, which approximates a power law. Continuum radiation returning to the disc because of the Kerr metric does not enhance the equivalent width of the line seen above 3 keV by more than about 20 per cent if the continuum source corotates with the disc. © 1997 RAS.