NuSTAR observations of water megamaser AGN

Abstract

Aims: We 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 the available physical quantities from VLBI mapping. Results: Most of the sources that we analyzed are heavily obscured, showing a column density in excess of ~10<SUP>23</SUP> cm<SUP>-2</SUP>; in particular, 79% are Compton-thick (N<SUB>H</SUB> > 1.5 × 10<SUP>24</SUP> cm<SUP>-2</SUP>). When 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, we can predict the torus dimensions. 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 that connect 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.