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

Abstract

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 $\sim 3\sigma$, the other fluorescent lines have a significance between 2 and 2.5 $\sigma$, while the Cr line has a significance of $\sim 1.5\sigma$. 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 $N_{\mathrm{H}}\sim 7\times 10^{24}$ 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.

5 pages, 5 figures, to appear in MNRAS Letter, some typos added

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• Hard X-ray Irradiation Potentially Drives Negative AGN Feedback by Altering Molecular Gas Properties
• The clumpy torus around type II AGN as revealed by X-ray fluorescent lines
• Revisit the Circumnuclear X-ray Emission of NGC 2992 in a Historically Low State
• Spectral Modeling of Charge Exchange in the Central Region of M51
• Canonical X-Ray Fluorescence Line Intensities as Column Density Indicators
• On the emitting region of X-ray fluorescent lines around Compton-thick AGN
• Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations