  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.
