X-ray fluorescent lines from the Compton-thick AGN in M51
  The cold disk/torus gas surrounding active galactic nuclei (AGN) emits
fluorescent lines when irradiated by hard X-ray photons. The fluorescent lines
of elements other than Fe and Ni are rarely detected due to their relative
faintness. We report the detection of K$\alpha$ lines of neutral Si, S, Ar, Ca,
Cr, and Mn, along with the prominent Fe K$\alpha$, Fe K$\beta$, and Ni
K$\alpha$ lines, from the deep Chandra observation of the low-luminosity
Compton-thick AGN in M51. The Si K$\alpha$ line at 1.74 keV is detected at
$\sim3\sigma$, the other fluorescent lines have a significance between 2 and
2.5 $\sigma$, while the Cr line has a significance of $\sim1.5\sigma$. These
faint fluorescent lines are made observable due to the heavy obscuration of the
intrinsic spectrum of M51, which is revealed by Nustar observation above 10
keV. The hard X-ray continuum of M51 from Chandra and Nustar can be fitted with
a power-law spectrum with an index of 1.8, reprocessed by a torus with an
equatorial column density of $N_{\rm H}\sim7\times10^{24}$ cm$^{-2}$ and an
inclination angle of $74$ degrees. This confirms the Compton-thick nature of
the nucleus of M51. The relative element abundances inferred from the fluxes of
the fluorescent lines are similar to their solar values, except for Mn, which
is about 10 times overabundant. It indicates that Mn is likely enhanced by the
nuclear spallation of Fe.
