Accurate polarization calibration of FAST spectral data for measurements of Zeeman splittings of OH megamasers in IRAS 02524+2046

Abstract

An accurate polarization calibration is essential for a spectral data analysis and Zeeman splitting measurements. Two anomalies challenge our understanding of OH megamasers in IRAS 02524+2046: an unexplained 1667/1665 MHz flux-ratio deviation, and complex Stokes V signatures. Well-calibrated sensitive polarization observations are required to understand them. We develop a polarization calibration solution for the L-band 19-beam receiver installed on the Five-hundred-meter aperture spherical radio telescope (FAST) to achieve a high calibration accuracy and thus enable accurate measurements of the OH megamaser properties in IRAS 02524+2046. We determined the Mueller matrix solution for spectral observations across the 1050-1450 MHz frequency range with an accuracy of about 0.01%-0.08% for circular polarization. We then applied it to FAST observational data of IRAS 02524+2046. Our results show narrower emission line components in the OH megamasers than previously reported, which are indistinguishable in the total power spectrum, but are detected in the circular polarization spectrum. The 1667 MHz OH megamaser emissions probably span a wide velocity range from ~54750 to ~53580 km/s, indicating greater complexity than previously recognized. Our fit of the total power and circular polarization spectra for IRAS 02524+2046 revealed ten line components with significant Zeeman splitting (>3sigma), indicating in situ magnetic fields with a strength of approximately -24.5 mG to +20.6 mG, most of which (8/10) have positive values.