Fe XXV and Fe XXVI lines from low velocity, photoionised gas in the X-ray spectra of AGN

Abstract

We have calculated the equivalent widths of the absorption lines produced by Fe XXV and Fe XXVI in a Compton-thin, low-velocity photoionised material illuminated by the nuclear continuum in AGN. The results, plotted against the ionisation parameter and the column density of the gas, are a complement to those presented by Bianchi & Matt 2002 for the emission lines from the same ionic species. As an extension to the work by Bianchi & Matt 2002, we also present a qualitative discussion on the different contributions to the He-like iron emission line complex in the regimes where recombination or resonant scattering dominates, providing a useful diagnostic tool to measure the column density of the gas. Future high resolution missions (e.g., Astro-E2) will allow us to fully take advantage of these plasma diagnostics. In the meantime, we compare our results with an up-to-date list of Compton-thick and unobscured (at least at the iron line energy) Seyfert galaxies with emission and/or absorption lines from H- and He-like iron observed with Chandra and XMM-Newton.