A Polarization Pipeline for Fast Radio Bursts Detected by CHIME/FRB

Abstract

Polarimetric observations of Fast Radio Bursts (FRBs) are a powerful resource for better understanding these mysterious sources by directly probing the emission mechanism of the source and the magneto-ionic properties of its environment. We present a pipeline for analysing the polarized signal of FRBs captured by the triggered baseband recording system operating on the FRB survey of The Canadian Hydrogen Intensity Mapping Experiment (CHIME/FRB). Using a combination of simulated and real FRB events, we summarize the main features of the pipeline and highlight the dominant systematics affecting the polarized signal. We compare parametric (QU-fitting) and non-parametric (rotation measure synthesis) methods for determining the Faraday rotation measure (RM) and find the latter method susceptible to systematic errors from known instrumental effects of CHIME/FRB observations. These errors include a leakage artefact that appears as polarized signal near $\mathrm{RM}\sim 0\mathrm{rad}{\mathrm{m}}^{-2}$ and an RM sign ambiguity introduced by path length differences in the system's electronics. We apply the pipeline to a bright burst previously reported by \citet[FRB 20191219F;][]{Leung2021}, detecting an $\mathrm{RM}$ of $+6.074\pm 0.006\pm 0.050\mathrm{rad}{\mathrm{m}}^{-2}$ with a significant linear polarized fraction ($\gtrsim 0.87$) and strong evidence for a non-negligible circularly polarized component. Finally, we introduce an RM search method that employs a phase-coherent de-rotation algorithm to correct for intra-channel depolarization in data that retain electric field phase information, and successfully apply it to an unpublished FRB, FRB 20200917A, measuring an $\mathrm{RM}$ of $-1294.47\pm 0.10\pm 0.05\mathrm{rad}{\mathrm{m}}^{-2}$ (the second largest unambiguous RM detection from any FRB source observed to date).

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