Maser polarization through anisotropic pumping
In one sentence
A unified maser-polarization code — the first to model anisotropic pumping, magnetic-field effects, and full saturation behaviour simultaneously — disentangling how much of an observed signal is set by the magnetic field, how much is boosted by the geometry of how the maser is pumped, and where the pumping can produce non-Zeeman circular polarization that masquerades as a magnetic signature.
What’s the question?
Cosmic masers — natural radio amplifiers in molecules like CH₃OH, H₂O and SiO — are often strongly polarized. The magnetic field, via the Zeeman effect, sets the direction of that polarization, but the magnetic field alone struggles to account for its strength: SiO masers in particular routinely show linear polarization fractions of tens of percent, far in excess of what a typical interstellar field would produce on its own. The missing ingredient is anisotropic pumping — when the radiation that excites the maser comes from a preferred direction, the upper level of the maser transition is aligned, and the polarization is boosted well above the bare-Zeeman level (while the magnetic field continues to set the polarization angle). The same anisotropy can, when the magnetic-field direction changes along the propagation through the maser, also produce circular polarization that masquerades as a genuine Zeeman signal. Neither effect had been modelled comprehensively alongside the rest of the maser physics.
What did we do?
I combined polarization-resolved excitation modelling with full polarized maser radiative transfer and wrote a single code that does both at once. I computed anisotropic-pumping parameters for class I CH₃OH, H₂O, and SiO masers, and showed that anisotropic pumping naturally explains why SiO masers are so strongly polarized while other species are more modest. The framework also predicts a previously unrecognised mechanism for non-Zeeman circular polarization, generated whenever the magnetic field rotates direction along the propagation through an anisotropically pumped maser.
Why does it matter?
A meaningful fraction of the polarization that observers attribute to magnetic fields in masers is, on this analysis, set by pumping geometry — meaning some published field strengths inferred from masers need to be revisited. More positively, the same framework gives a quantitative way to use the pumping geometry as its own diagnostic.
My role
First author. Devised the method, wrote the code, performed the simulations, and wrote the paper.