X-ray fluorescent lines from the Compton-thick AGN in M51

The cold disc/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 Ka, Fe K beta, and Ni K alpha lines, from the deep Chandra observation of the low-luminosity Compton-thick AGN in M51. The Si Ka line at 1.74 keV is detected at similar to 3 sigma, the other fluorescent lines have a significance between 2 and 2.5 sigma, while the Cr line has a significance of similar to 1.5 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-H similar to 7 x 10(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.