Towards Solving the Origin of Circular Polarisation in FRB 20180301A

Fast Radio Bursts (FRBs) are short-timescale transients of extragalacticorigin. The number of detected FRBs has grown dramatically since theirserendipitous discovery from archival data. Some FRBs have also been seen torepeat. The polarimetric properties of repeating FRBs show diverse behaviourand, at times, extreme polarimetric morphology, suggesting a complexmagneto-ionic circumburst environment for this class of FRB. The polarimetricproperties such as circular polarisation behaviour of FRBs are crucial forunderstanding their surrounding magnetic-ionic environment. The circularpolarisation previously observed in some of the repeating FRB sources has beenattributed to propagation effects such as generalised Faraday rotation (GFR),where conversion from linear to circular polarisation occurs due to thenon-circular modes of transmission in relativistic plasma. The discovery burstfrom the repeating FRB20180301A showed significant frequency-dependentcircular polarisation behaviour, which was initially speculated to beinstrumental due to a sidelobe detection. Here we revisit the properties giventhe subsequent interferometric localisation of the burst, which indicates thatthe burst was detected in the primary beam of the Parkes/Murriyang 20-cmmultibeam receiver. We develop a Bayesian Stokes-Q, U, and V fit method tomodel the GFR effect, which is independent of the total polarised fluxparameter. Using the GFR model we show that the rotation measure (RM) estimatedis two orders of magnitude smaller and opposite sign (∼28 radm^-2)than the previously reported value. We interpret the implication of thecircular polarisation on its local magnetic environment and reinterpret itslong-term temporal evolution in RM.