The rotation measure of background radio sources seen through the supernova remnant OA184 (G166.2 + 2.5)

Radio sources in the field of the extended SNR OA184 (G166.2 + 2.5) have been studied to determine the excess rotation measure (RM) arising from the SNR. Of a total of 32 radio sources observed with the VLA in the C configuration, eight are found to be polarized above 7 σ. The sources seen through the SNR show significantly high RM in comparison to background sources. The excess RM due to the Faraday active plasma in the SNR is estimated to be 150 ±20 rad m-2, which corresponds to left-angle neB-6 right-angle rim = (30 ±5)L10pc-1. The sign of RMs of the sources within an area of about 2 degrees x 2 degrees centered on the SNR shows a systematic longitudinal polarity change on either side of l ∼ 166 degrees.2. Although a larger sample is needed to justify this, the authors tentatively interpret this flip as due to the reversal of an irregular component of the galactic magnetic field on a scale of order 100 pc