Abstract
Strong H2O maser emission is detected from active galactic nuclei (AGN) as well as Galactic objects such as star-forming regions and late-type stars. In spite of the widely different luminosities of the different masers, a common pump mechanism seems adequate: neutral collisions at densities of ~ 108-1010 cm-3 and temperatures of ~ 250-500 K. The different properties of the various masers can be attributed to geometry. Although disk rotation controls the AGN maser geometry it does not directly determine the dimensions. X-ray radiation and spiral shocks have been suggested as the heating sources of H2O megamasers. Both are capable of explaining the observations, and the radiative scenario seems best understood in terms of chance alignment of standard broad-line-region clouds. It is not yet clear whether these different proposals produce distinct maser signatures.
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Elitzur, M. (1998). Pumping of H2O Megamasers. In: Andersen, J. (eds) Highlights of Astronomy. International Astronomical Union / Union Astronomique Internationale, vol 11B. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-4778-1_107
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DOI: https://doi.org/10.1007/978-94-011-4778-1_107
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