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

Circumnuclear water masers can respond in two ways to changes in the luminosity of an active galactic nucleus. First, an increase in the X-ray luminosity can lead to an increase in the maser emissivity, and second, an increase in the intrinsic bolometric luminosity may result in a temporary decrease in the difference between the gas and dust temperature and a consequent decrease in the maser output. While the latter effect can occur over a period shorter than the thermal timescale, the former effect cannot. Quantitative estimates of the response of the water maser emissivity to changes in either the X-ray or bolometric luminosity are presented, together with estimates of the relevant timescales. Either mechanism could account for recent observations by Gallimore et al. suggesting that the water maser variability in two widely separated regions of the circumnuclear gas in NGC 1068 have been coordinated by a signal from the active nucleus. For either mechanism, a minimum H₂ density of ~10⁸ cm^-3 is needed to explain the observed variability timescale.