On the origin of the iron K α line cores in active galactic nuclei

X-ray observations made with Chandra and XMM–Newton have shown that there are relatively narrow cores to the iron Kα emission lines in active galactic nuclei (AGN). Plausible origins for this core emission include the outer regions of an accretion disc, a parsec-scale molecular torus, and the optical broad-line region (BLR). Using data from the literature it is shown that no correlation exists between the Fe Kα core width and the BLR (specifically Hβ) line width. This shows that in general the iron Kα core emission does not arise from the BLR. There is a similar lack of correlation between the width of the Fe Kα core and black hole mass. The average Kα width is about a factor of 2 lower than the Hβ width. It therefore seems likely that, in many cases, the narrow core arises in the torus. There is a very wide range of observed Fe Kα core widths, however, and this argues for multiple origins. The simplest explanation for the observed line profiles in AGN is that they are due to a mixing of very narrow emission from the inner edge of the torus, and broadened emission from the accretion disc, in varying proportions from object to object.