been found to be almost ubiquitous (e.g., [@Nandra:2007ly], [@Singh:2011ly]). This component peaks at $6.4$keV (e.g.,[@Yaqoob:2004vn]), which points to the line being produced in cold neutral material. This material has often been identified as circumnuclear matter located at several thousand gravitational radii from the supermassive black hole, and is likely related to the putative molecular torus (e.g., [@Nandra:2006fk]), although a contribution of the outer part of the disk (e.g., [@Petrucci:2002fk]) or of the broad-line region (BLR, e.g., [@Bianchi:2008fk]) cannot be excluded. In a recent study, @Shu:2011fk found that the weighted mean of the ratio between the FWHM of the narrow Fe K$\alpha$ line and that of optical lines produced in the BLR is $\simeq 0.6$. This implies that the size of the iron K$\alpha$-emitting region is on average $\sim3$ times that of the BLR, and points towards most of the narrow iron K$\alpha$ emission being produced in the putative molecular torus.

[lcccc]{} & (2) & (3) & (4) & (5)\
Reference & $A$ & $B$ & Sample & Observatory/instrument\
\
[@Shu:2012fk] & $1.58\pm0.03^*$ & $-0.18\pm0.03$ & 32 (RQ) & [*Chandra*]{}/HEG\
[@Shu:2010zr] & $1.58\pm 0.03^*$ & $-0.22\pm0.03$ & 33 (RQ+RL) & [*Chandra*]{}/HEG\
[@Bianchi:2007vn] & $1.73\pm 0.03$ & $-0.17\pm0.03$ & 157 (RQ) & 