FLUCTUATIONS IN ASTRONOMICAL MASERS

The radiation of astronomical masers fluctuates on the time scale GAMMA--1, where GAMMA is the levels' loss rate. These intensity fluctuations reflect fluctuations of the level populations around their mean, steady state values over the length scale lambda-c = c/GAMMA. In saturated masers, the intensity fluctuations are dominated by passage through the unsaturated core. The effects of the saturated zones and of the seed radiation that the masers amplify can be neglected. These fluctuations may have been detected in recent observations by Clegg & Cordes of Galactic H II/OH masers, providing a possible direct determination of the saturation intensity J(s) (= GAMMA/2B), an important input parameter for theoretical modeling. Long time-scale variations in the fluctuation amplitudes can provide information on phenomena such as passage of hydrodynamic waves in the maser region.