A Four-Stokes-Parameter Spectral Line Polarimeter at the Caltech Submillimeter Observatory

TL;DR

The paper introduces a new four-Stokes-parameter spectral line polarimeter (FSPPol) designed for the Caltech Submillimeter Observatory, capable of measuring all polarization states with high reliability in the 200-260 GHz range.

Contribution

It presents the design, implementation, and initial observational results of FSPPol, a novel instrument that measures all four Stokes parameters without complex optical components.

Findings

Measured polarization fractions of a few percent in CO spectral lines.

First study of Zeeman effect in CN N=2→1 transition in DR21(OH).

Instrument demonstrates low polarization contamination.

Abstract

We designed and built a new Four-Stokes-Parameter spectral line Polarimeter (FSPPol) for the Caltech Submillimeter Observatory (CSO). The simple design of FSPPol does not include any mirrors or optical components to redirect or re-image the radiation beam and simply transmits the beam to the receiver through its retarder plates. FSPPol is currently optimized for observation in the 200-260 GHz range and measures all four Stokes parameters, I, Q, U, and V. The very low level of instrument polarization makes it possible to obtain reliable measurements of the Goldreich-Kylafis effect in molecular spectral lines. Accordingly, we measured a polarization fraction of a few percent in the spectral line wings of ^{12}\mathrm{CO} (J=2\rightarrow1) in Orion KL/IRc2, which is consistent with previous observations. We also used FSPPol to study the Zeeman effect in the N=2\rightarrow1 transition of CN in DR21(OH) for the first time. At this point we cannot report a Zeeman detection, but more observations are ongoing.