The iron Kα line from a partially ionized reflecting medium in an active galactic nucleus

Abstract

We perform Monte Carlo simulations of the reflection of X-rays from a partially ionized medium. Properties of the reflected continuum and iron Ka line are investigated both for irradiated slabs and for accretion discs. In the latter case we study the line emission for two prescriptions for viscosity ('α' and 'β' discs) and a point source of irradiation as a function of the accretion rate. We find that under the influence of external illumination 'α' discs can be significantly ionized and the equivalent width of the line can be much higher than in the case of a neutral medium, reaching a maximum value of 270 eV, although much lower values are possible due either to resonant trapping of line photons or to complete ionization of iron. We compute the dependence of the emission on the incident angle of ingoing radiation and the inclination angle of observation. We compare our results with previous calculations of iron Ka line emission due to reflection from a partially ionized medium carried out using different methods. Finally, we discuss the applicability of our results to Seyfert 1 galaxies with exceptionally strong iron Kα emission.