On the influence of resonant absorption on the iron emission line profiles from accreting black holes

Abstract

The fluorescent iron K$\alpha$ emission line profile provides an excellent probe of the innermost regions of active galactic nuclei. Fe XXV and Fe XXVI in diffuse plasma above the accretion disc can affect the X-ray spectrum by iron K$\alpha$ resonant absorption. This in turn can influence the interpretation of the data and the estimation of the accretion disc and black hole parameters. We embark on a fully relativistic computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly-absorbing plasma occurs close to the black hole. This can explain the features seen in the iron K$\alpha$ line profile recently obtained by Nandra et al. (1999) for the Seyfert 1 galaxy NGC 3516. We show that the redshift of this feature can be mainly gravitational in origin and accounted for without the need to invoke fast accretion of matter onto the black hole. New X-ray satellites such as XMM, ASTRO-E and Chandra provide excellent opportunities to test the model against high quality observational data.