NuSTAR observations of water megamaser AGN

TL;DR

This study uses NuSTAR X-ray observations of 14 water megamaser AGN to explore the relationship between the masing disk and the obscuring torus, revealing that most are heavily obscured and that the maser disk is connected to the inner torus, consistent with IR observations.

Contribution

It provides a unified X-ray spectral analysis linking maser disks with the obscuring torus, proposing a model where the maser disk is part of the torus structure based on spectral and VLBI data.

Findings

Most sources are heavily obscured, with 79% being Compton-thick.

Torus dimensions predicted from column densities match IR interferometry observations.

The maser disk is likely composed of molecular clouds connecting to the outer accretion disk.

Abstract

Aims. Study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser Active Galactic Nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with available physical quantities from VLBI mapping. Results. Most of the sources analyzed are heavily obscured, showing a column density in excess of $\sim 10^{23}$ cm$^{-2}$. In particular, $79\%$ are Compton-thick ($N_{\rm H} > 1.5 \times 10^{24}$ cm$^{-2}$). Using column densities measured by NuSTAR, with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, the torus dimensions can be predicted. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds connecting the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.