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C. Heiles (UC, Berkeley)
The newly finished Arecibo Observatory is ideal for measuring Zeeman splitting of the 21-cm line in emission and absorption. We describe our measurements of instrumental parameters and new detections made during an observing run in February/March 1999. The essence is measuring the Stokes V parameters, which is the difference between the two circular polarizations. In all previous measurements this difference has been obtained directly by polarization switching. At Arecibo we obtained it by a far superior technique: cross-correlation of two linear polarizations. This technique is superior for two reasons: (1) one obtains all four Stokes parameters simultaneously; (2) polarization purity can be measured and calibrated to exquisite precision. We find beam squint (the difference in direction between the two circular polarizations) to be 0.7'', about three times smaller than the best previously-used telescope; this corresponds to a typical instrumental contribution to measured field strength of 0.2 microG, and even this small value can be reduced by corrective techniques. We describe three new detections of Zeeman splitting and discuss their astrophysical significance. Finally, we discuss the first detection of linear polarization in 21-cm absorption lines. This work was supported in part by an NSF grant to the author.
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