The polarization of maser radiation when the source is permeated by an aligned magnetic field is derived for arbitrary angular momenta of the transition states. This generalization is made possible by an analysis of the structure of the propagating waves in a frame aligned with the magnetic axis. The key elements in determining the polarization properties are the assumption of independent and incoherent pump and loss processes for all magnetic sublevels, and the beaming of maser radiation. The radiation propagating in the direction of maximal intensity growth is polarized according to the solutions derived by Goldreich, Keeley, and Kwan for J = 1 → 0 masers, using various assumptions about the pump rates and the relation between line width and Zeeman splitting. These solutions are shown to be the most general ones for the dominant rays (those propagating along the longest chords through the source) of steady state beamed masers with arbitrary spins in the relevant domains of parameter space. The beamed nature, and polarization properties, of astronomical maser radiation are established during the unsaturated growth phase, when an increase in overall source dimension results in proportionate tightening of the beaming factor Ω/4π.

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Published in The Astrophysical Journal, v. 370, no. 1, p. 407-418.

©1991. The American Astronomical Society. All rights reserved.

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