I was reading that:

However, unlike Galactic sources such as W3(OH), the emission is unpolarized and the 1667 MHz line is stronger than the 1665 MHz line.

but how is this possible? Does not the 1665 MHz line and 1667 MHz line correspond to a particular transition and dipole moment? Won't emitted photons naturally have a polarization?

Furthermore, they state:

The characteristics of the λ18 cm OH mega-maser emission differ from those of Galactic maser sources, such as the main line intensity ratio (T1667MHz/T1665 MHz > 1), large linewidth (>100 km s^−1), and unpolarized emission.

But why is unpolarized emission expected from extragalactic masers (arising near AGN) and not galactic ones (arising in circumstellar and interstellar environments)?

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    $\begingroup$ I don't know about these sources in particular, but I think you can say that in order to be polarized, something special has to happen. For example, ALL the radiation is from stimulated emission started by a single photon. Or there is some physical phenomenon that polarizes the radiation. As a laboratory example, a gas LASER uses "Brewster windows" so that only light polarized in one direction can reflect back and forth in the tube and produce more emission. It is probably easier to get polarized MASER by things like magnetic fields, due to the gross nature of the emitters. $\endgroup$ – C. Towne Springer Feb 18 '17 at 5:45
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    $\begingroup$ @C.TowneSpringer that sounds right to me. Laboratory lasers can often be bought as polarized, or unpolarized, but if you look closely the unpolarized ones are really "not intentionally polarized" and if you made high speed measurements you might find a high degree of polarization who's orientation was jumping unpredictably with time. These naturally occurring sources are so "multimode" compared to lab sources just about anything can happen as far as polarization. $\endgroup$ – uhoh Feb 18 '17 at 10:56

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