Maser Theory

This review covers some of the developments in basic theory of astronomical masers over the past ten years. Topics included are the effects of three dimensional geometry and polarization, with special emphasis on the differences between maser and non-maser radiation. 1. Maser Absorption Coefficient in Three Dimensions The absorption coefficient κ describes the coupling between particles and radiation and serves as the foundation of maser theory. The appropriate expression, taking proper care of the Doppler matching between particles with thermal velocity distribution and photons with a given wave vector k (aligned with the z-axis) and frequency ν, was developed by Litvak (1973). It can be written as κ(k) = c ν0 ∫ κ0(vk) 1 + I(vk)/Js dvxdvy , (1)