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Circumnuclear water masers can respond in two ways to changes in theluminosity of an active galactic nucleus. First, an increase in the X-rayluminosity can lead to an increase in the maser emissivity; and second, anincrease in the intrinsic bolometric luminosity may result in a temporarydecrease in the difference between the gas and dust temperature and aconsequent decrease in the maser output. Whilst the latter effect can occurover a period shorter than the thermal timescale, the former effect cannot.Quantitative estimates of the response of the water maser emissivity to changesin either the X-ray or bolometric luminosity are presented, together withestimates of the relevant timescales. Either mechanism could account for recentobservations by Gallimore et al. which suggest that the water maser variabilityin two widely separated regions of the circumnuclear gas in NGC 1068 have beencoordinated by a signal from the active nucleus. For either mechanism, aminimum H2 density ~ 1.E+8 cm-3 is needed to explain the observed variabilitytimescale.Comment: 12 pages (AASTeX), including 4 figures. Accepted for publication in  ApJ Letter

Response of Circumnuclear Water Masers to Luminosity Changes in an Active Galactic Nucleus