On the emitting region of X-ray fluorescent lines around Compton-thick AGN

TL;DR

This study measures the intrinsic widths of Si Ka X-ray fluorescent lines in Compton-thick AGN, revealing that the line-emitting regions are outside the dust sublimation radius and likely coincide with the dusty torus.

Contribution

It provides the first high-resolution measurements of Si Ka line widths in AGN, showing they are narrower than Fe Ka lines and refining the understanding of the emitting regions.

Findings

Si Ka line widths are 3-5 times smaller than Fe Ka lines.

Line-emitting regions are outside the dust sublimation radius.

X-ray fluorescent material likely coincides with dusty torus.

Abstract

X-ray fluorescent lines are unique features of the reflection spectrum of the torus when irradiated by the central AGN. Their intrinsic line width can be used to probe the line-emitting region. Previous studies have focused on the Fe Ka line at 6.4 keV, which is the most prominent fluorescent line. These studies, however, are limited by the spectral resolution of currently available instruments, the best of which is $\sim1860$ km s$^{-1}$ afforded by the Chandra High-Energy Grating (HEG). The HEG spectral resolution is improved by a factor of 4 at 1.74 keV, where the Si Ka line is located. We measured the FWHM of the Si Ka line for Circinus, Mrk 3, and NGC 1068, which are $570\pm240$, $730\pm320$, and $320\pm280$ km s$^{-1}$, respectively. They are $3-5$ times smaller than those measured with the Fe Ka line previously. It shows that the intrinsic widths of the Fe Ka line are most likely to be over-estimated. The measured widths of the Si Ka line put the line-emitting region outside the dust sublimation radius in these galaxies. It indicates that for Compton-thick AGN, the X-ray fluorescence material are likely to be the same as the dusty torus emitting in the infrared.