Towards solving the origin of circular polarization in FRB 20180301A

ABSTRACT Fast radio bursts (FRBs) are short-time-scale transients of extragalactic origin. The number of detected FRBs has grown dramatically since their serendipitous discovery from archival data. Some FRBs have also been seen to repeat. The polarimetric properties of repeating FRBs show diverse behaviour and, at times, extreme polarimetric morphology, suggesting a complex magneto-ionic circumburst environment for this class of FRB. The polarimetric properties such as circular polarization behaviour of FRBs are crucial for understanding their surrounding magnetic-ionic environment. The circular polarization previously observed in some of the repeating FRB sources has been attributed to propagation effects such as generalized Faraday rotation (GFR), where conversion from linear to circular polarization occurs due to the non-circular modes of transmission in relativistic plasma. The discovery burst from the repeating FRB 20180301A showed significant frequency-dependent circular polarization behaviour, which was initially speculated to be instrumental due to a sidelobe detection. Here, we revisit the properties given the subsequent interferometric localization of the burst, which indicates that the burst was detected in the primary beam of the Parkes/Murriyang 20-cm multibeam receiver. We develop a Bayesian Stokes-Q, U, and V fit method to model the GFR effect, which is independent of the total polarized flux parameter. Using the GFR model we show that the rotation measure (RM) estimated is two orders of magnitude smaller and opposite sign ($\sim$28 rad m$^{-2}$) than the previously reported value. We interpret the implication of the circular polarization on its local magnetic environment and reinterpret its long-term temporal evolution in RM.