The intrinsic line width of the Fe Ka line of AGN

TL;DR

This study refines measurements of the Fe Kα line widths in AGN, demonstrating that previous overestimations occurred with first-order data, and confirms the line's origin near the cold dusty torus.

Contribution

Using higher-resolution second and third order Chandra HEG spectra, the paper shows that the intrinsic widths of the Fe Kα line are smaller than previously measured, clarifying its emission region.

Findings

Fe Kα line widths are over-estimated in first order data.

Higher-resolution spectra reveal smaller, more accurate line widths.

Fe Kα lines originate near the cold dusty torus.

Abstract

X-ray fluorescent lines are unique features of the reflection spectrum of the cold torus when irradiated by the central AGN. Their intrinsic line widths can be used to probe the line-emitting region. The line widths of the Fe \Ka line measured from the first order Chandra High Energy Grating (HEG) spectra are $3-5$ times larger than those measured with the Si \Ka line for Circinus, Mrk 3, and NGC 1068. Because the observed Si \Ka and Fe \Ka lines are not necessarily coming from the same physical region, it is uncertain whether the line widths of the Fe \Ka line are over-estimated or not. We measured the intrinsic line widths of the Fe \Ka line of several nearby bright AGN using the second and third order Chandra HEG spectra, whose spectral resolutions are better than the first order data. We found the measured widths are all smaller than those from the first order data. The results clearly show that the widths of the Fe \Ka line measured from the first order HEG data are over-estimated. It indicates that the Fe \Ka lines of the studied sources are originating from regions around the cold dusty torus.