SIMBAD references

The Fe XXII doublet has been previously used to determine the density of collisionally ionized emission from magnetic cataclysmic variable stars. We test how this diagnostic doublet behaves for a photoionized plasma with the spectral energy distribution (SED) of an active galactic nucleus (AGN). We use the photoionized plasma code PION and ∼440 ks of archival Chandra High Energy Transmission Grating observations for the well-known Seyfert 2 galaxy NGC 1068 to test the behaviour of the Fe XXII doublet in the context of an AGN. This marks the first time these data have been examined with PION. We find that in a photoionized plasma, the Fe XXII doublet is dependent on the density, ionization state, and SED used. Thus, this density diagnostic remains model-dependent. In the context of NGC 1068 the doublet predicts an emission region ∼100r_g from the central black hole. This would require a direct line of sight to the central engine, which is at odds with the Seyfert 2 nature of this source. In practice, these results highlight the complexities and challenges of applying photoionized models. With these data, we cannot exclude the possibility of a direct line of sight to the central engine of NGC 1068, but we cannot confirm it. Future observations with instruments such as Athena are needed to explore the Fe XXII doublet further.