THOR Polarimetry as a Means to Probe the Magnetic Field Structure of Supernova Remnants and the Milky Way

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The Galactic magnetic field is composed of a complex structure, many aspects of which are debated and still active areas of research. This thesis presents the first polarimetry results of the THOR (The HI, OH, Radio recombination line) survey of the Milky Way. In the Galactic longitude range 39 degrees < l < 52 degrees, we find rotation measures (RMs) in the range -310 rad m^{-2} < RM < +4219 rad m^{-2}, with the highest values concentrated within a degree of l = 48 degrees at the Sagittarius arm tangent. Most of the high RMs arise in diffuse plasma, along lines of sight that do not intersect HI regions. For l > 49 degrees, RM drops off rapidly, while at l < 47 degrees, the mean RM is higher with a larger standard deviation than at l > 49 degrees. We attribute the RM structure to the compressed diffuse warm ionized medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of fast radio bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys. We present polarization and Faraday rotation for the supernova remnants (SNRs) G46.8-0.3, G43.3-0.2, G41.1-0.3, and G39.2-0.3 in the L-band (1-2 GHz) radio continuum in the HI/OH/Recombination line survey. We detect polarization from G46.8-0.3, G43.3-0.2, and G39.2-0.3, but find upper limits at the 1% level of Stokes I for G41.1-0.3. For G46.8-0.3 and G39.2-0.3, the fractional polarization varies on small scales from 1% to ~6%. G43.3-0.2 is less polarized with fractional polarization <~3%. We find upper limits at the 1% level for the brighter regions in each SNR with no evidence for associated enhanced Faraday depolarization. We observe significant variation in Faraday depth and fractional polarization on angular scales down to the resolution limit of 16''. Approximately 6% of our polarization detections from G46.8-0.3 and G39.2-0.3 exhibit two-component Faraday rotation and 14% of polarization detections in G43.3-0.2 are multicomponent. For G39.2-0.3, we find a bimodal Faraday depth distribution with a narrow peak and a broad peak for all polarization detections as well as for the subset with two-component Faraday rotation. We identify the narrow peak with the front side of the SNR and the broad peak with the back side. Similarly, we interpret the observed Faraday depth distribution of G46.8-0.3 as a superposition of the distributions from the front side and the back side. We interpret our results as evidence for a partially filled shell with small-scale magnetic field structure and internal Faraday rotation. We then present the continued analysis of polarization and Faraday rotation for the supernova remnants (SNRs) G46.8-0.3 and G39.2-0.3. In this work, we present our investigation of Faraday depth fluctuations from angular scales comparable to the size of the SNRs down to scales less than our 16'' beam (<~0.7 pc) from Faraday dispersion (sigma_phi). With THOR polarization, we find median sigma_phi of 15.9 +/- 3.2 rad m^{-2} for G46.8-0.3 and 17.6 +/- 1.6 rad m^{-2} for G39.2-0.3. When comparing to polarization at 6 cm, we find evidence for sigma_phi >~ 30 rad m^{-2} in localized regions where we detect no L-band polarization in THOR. We combine Faraday depth dispersion with the rotation measure (RM) structure function (SF) and find evidence for a break in the SF on scales less than the THOR beam. We estimate the RM SF of the foreground interstellar medium using the SF of extragalactic radio sources and pulsars to find that the RM fluctuations we measure originate within the SNRs for all but the largest angular scales.
Galactic magnetism, Magnetism, Supernova remnant, Supernova, Galactic magnetic field, Magnetic field, Milky Way, Supernova remnants, Interstellar medium
Shanahan, R. P. (2024). THOR polarimetry as a means to probe the magnetic field structure of supernova remnants and the Milky Way (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from