A long hard-X-ray look at the dual active galactic nuclei of M51 with NuSTAR

TL;DR

This study provides a detailed X-ray spectral analysis of the M51 galaxy system, revealing the obscured nature and low accretion rates of its dual AGN, and examines off-nuclear sources with spectral turnovers, challenging existing theories on AGN obscuration.

Contribution

It offers the first broadband X-ray spectral analysis of M51's dual AGN using NuSTAR and Chandra, showing low accretion rates and persistent obscuration at low luminosities.

Findings

M51a's AGN is Compton thick with very low accretion rate.

Obscuring material's covering factor aligns with local AGN obscured fraction.

Most off-nuclear sources exhibit spectral turnover below 10 keV.

Abstract

We present a broadband X-ray spectral analysis of the M51 system, including the dual active galactic nuclei (AGN) and several off-nuclear point sources. Using a deep observation by NuSTAR, new high-resolution coverage of M51b by Chandra, and the latest X-ray torus models, we measure the intrinsic X-ray luminosities of the AGN in these galaxies. The AGN of M51a is found to be Compton thick, and both AGN have very low accretion rates ($\lambda_{\rm Edd} <10^{-4}$). The latter is surprising considering that the galaxies of M51 are in the process of merging, which is generally predicted to enhance nuclear activity. We find that the covering factor of the obscuring material in M51a is $0.26 \pm 0.03$, consistent with the local AGN obscured fraction at $L_{\rm X}\sim 10^{40}$ erg s$^{-1}$. The substantial obscuring column does not support theories that the torus, presumed responsible for the obscuration, disappears at these low accretion luminosities. However, the obscuration may have resulted from the gas infall driven by the merger rather than the accretion process. We report on several extra-nuclear sources with $L_{\rm X}>10^{39}$ erg s$^{-1}$ and find that a spectral turnover is present below 10 keV in most such sources, in line with recent results on ultraluminous X-ray sources.