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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. $\endgroup$ Commented Feb 18, 2017 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
    Commented Feb 18, 2017 at 10:56

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The polarisation of masers is associated with Zeeman components produced by magnetic fields threading the masing medium. The level of polarisation and splitting of the Zeeman components tells you something about the magnetic field strength and orientation.

In extragalactic megamasers, at much greater distance than their galactic counterparts, individual masers are unlikely to be resolved. The light is probably a sum over numerous unresolved sources with their own physical conditions and magnetic fields. Thus the polarisation signature is absent or much weaker.

Polarisation has though been detected from megamasers, when looked for carefully. For example McBride & Heiles (2013) were able to detect a (weak) polarisation signature in about half the OH megamasers they studied.

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