The Cores of the Fe Kα Lines in Active Galactic Nuclei: an Extended Chandra High Energy Grating Sample

We extend the study of the core of the Fe Kα emission line at ∼6.4 keV in Seyfert galaxies reported in Yaqoob Padmanabhan (2004) using a larger sample observed by the Chandra High Energy Grating (HEG). Whilst heavily obscured active galactic nuclei (AGNs) are excluded from the sample, these data offer some of the highest precision measurements of the peak energy of the Fe Kα line, and the highest spectral resolution measurements of the width of the core of the line in unobscured and moderately obscured (N_H<10^23 cm^-2) Seyfert galaxies to date. The Fe Kα line is detected in 33 sources, and its centroid energy is constrained in 32 sources. In 27 sources the statistical quality of the data is good enough to yield measurements of the FWHM. We find that the distribution in the line centroid energy is strongly peaked around the value for neutral Fe, with over 80 giving values in the range 6.38–6.43 keV. Including statistical errors, 30 out of 32 sources (∼ 94 keV. The mean equivalent width, amongst the observations in which a non-zero lower limit could be measured, was53 ±3eV. The mean FWHM from the subsample of 27 sources was 2060 ± 230 km s^-1. The mean EW and FWHM are somewhat higher when multiple observations for a given source are averaged. From a comparison with the Hβ optical emission-line widths (or, for one source, Brα), we find that there is no universal location of the Fe Kα line-emitting region relative to the optical BLR. We confirm the presence of the X-ray Baldwin effect, an anti-correlation between the Fe Kα line EW and X-ray continuum luminosity. The HEG data have enabled isolation of this effect to the narrow core of the Fe Kα line.