A water maser flare in W49N: Amplification by a rotating foreground cloud

We monitored the 22 GHz H2O maser emission toward W49N from December 1989 t hrough May 1990. During this period we observed an outburst in a component at -66.25 km s(-1). The flux density of the flaring component increased by a factor of greater than or similar to 10 to a maximum of 4020 Jy over a pe riod of 24 days and decreased over the following 34 days to 1400 Jy on the last day of monitoring. During the flux increase the line narrowed from abo ut 1.1 to 0.8 k s(-1); it subsequently rebroadened to 1.0 km s(-1) Most int erestingly, during the flaring behavior the line center shifted by approxim ately 0.5 km s(-1) over the 58 day period. To explain the flare, particular ly its shifting line center, we present a model of two interacting maser cl ouds. In the model, saturated maser radiation produced in a background clou d is amplified by an unsaturated, masing foreground cloud. Motion of the fo reground cloud across the line of sight produces a flaring line, accompanie d by line narrowing. We demonstrate that the observed flare in W49N may be explained by such a model where a rotating foreground cloud passes in front of a non-rotating background cloud. The differential amplification of the background cloud's radiation produces the observed increase in flux density , line narrowing, and systematic shift in center velocity.