Prevalence of High X-ray Obscuring Columns among AGN that Host H$_2$O Masers

TL;DR

This study finds that most AGN hosting H$_2$O masers have high X-ray obscuring columns, especially in disk-maser systems, indicating a strong link between maser emission and Compton-thick obscuration.

Contribution

It provides the first comprehensive analysis of X-ray column densities in a large sample of AGN with H$_2$O masers, highlighting the high prevalence of Compton-thick obscuration.

Findings

95% of AGN with H$_2$O masers have N$_{ m H} >10^{23}$ cm$^{-2}$

60% exhibit N$_{ m H} >10^{24}$ cm$^{-2}$

76% of disk-maser AGN are Compton-thick

Abstract

Of 104 AGN known to exhibit H$_2$O maser emission, X-ray data that enable estimation of column densities, or lower limits, are available for 42. Contributing to this, we report analysis of new and archival X-ray data for 8 galaxies and collation of values for three more. Maser emission is indicative of large columns of cold gas, and in five of the eight new cases, maser spectra point toward origins in accretion disks viewed close-to edge-on (a.k.a. "disk-maser" systems). In these, we detect hard continuum and Fe K$\alpha$ emission with equivalent widths on the order of 1 keV, which is consistent with Compton reflection, fluorescence by cold material, and obscuring columns $\ga 10^{24}$ cm$^{-2}$. Reviewing the full sample of 42, 95% exhibit N$_{\rm H} >10^{23}$ cm$^{-2}$ and 60% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. Half of these are now recognized to be disk masers (up from 13); in this sub-sample, which is likely to be more homogeneous vis-\'a-vis the origin of maser emission, 76% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. The probability of a common parent distribution of columns for disk-masers and other AGN masers is <3%. Because ground-based surveys of AGN to detect new disk masers are relatively unbiased with respect to X-ray brightness and comparatively inexpensive, they may also be efficient guides for the sensitive pointed X-ray observations required to identify Compton-thick objects outside of shallow surveys.