On the origin of the iron Kα 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 K alpha 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 K alpha core width and the BLR (specifically H beta) line width. This shows that in general the iron K alpha core emission does not arise from the BLR. There is a similar lack of correlation between the width of the Fe K alpha core and black hole mass. The average K alpha width is about a factor of 2 lower than the H beta 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 K alpha 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.