He-like triplets as X-ray plasma diagnostics in AGN

The He-like triplet lines intervene in the calculation of the R and G line-ratios, which are used as plasma diagnostics to probe the physical state and to constrain the geometry of X-ray emitting regions. Although widely employed by the astrophysical community, these line-ratios should be addressed with some caution, as the He-like triplet intensities depend strongly on several aspects - both on the physical parameters characterizing the ionized medium and the computational methods used to model it. Here we briefly address some of them, such as the ionized gas conditions and the direction through which the line-emitting medium is being observed. For instance, the R and G measured on thick, stratified media in pressure equilibrium are not the same as the currently available diagnostics, designed for the more restricted case of constant density, thin media. Also, the R and G line-ratios vary with the observing angle; therefore, one should avoid the indiscriminate use of the same plasma diagnostics in type 1 and type 2 Active Galactic Nuclei.