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Probing Accretion Disks in AGN with Water Masers

Published online by Cambridge University Press:  12 April 2016

Philip R. Maloney
Philip R. Maloney*
Affiliation:
Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309–0389

Abstract

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Extremely luminous extragalactic water masers – the so-called “megamasers”, with isotropic luminosities of tens to hundreds of solar luminosities – appear to be uniquely associated with active galactic nuclei. The recent survey of Braatz et al. indicates that 20% of Seyfert 2 galaxies have detectable water maser emission. Although originally suggested to arise in shocks, it now seems likely that the masers arise from the irradiation of high-pressure molecular gas by X-rays from the AGN. Quantitative modelling shows that the observed megamaser luminosities can plausibly be produced in this fashion. Both observational limits on the size scales and the high gas pressures required indicate that the water maser emission arises on very small scales, either in a circumnuclear “torus” or the accretion disk itself. In the best-studied case, NCG 4258, the masers are produced in a geometrically thin, warped accretion disk. The maser models can be used to derive quantitative information about the physical conditions in the disk, namely, the mass accretion rate, and therefore the radiative efficiency. I discuss the implications of water maser observations and models for the study of accretion disks and circumnuclear tori in AGN.

Type
Part 3. Fundamental Physical Processes
Information
International Astronomical Union Colloquium , Volume 163: Accretion Phenomena and Related Outflows , 1997 , pp. 113 - 125
Copyright
Copyright © Astronomical Society of the Pacific 1997

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