Polarized Emission from Four Supernova Remnants in the THOR Survey

Abstract

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 H i/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.

Turbulent Structure in Supernova Remnants G46.8−0.3 and G39.2−0.3 from THOR Polarimetry

“…Radio frequency interference (RFI) flagging as well as standard flux and phase calibration of the continuum data are described in Beuther et al (2016). Standard procedures were followed for the polarization calibration and imaging done in CASA as described in Shanahan et al (2022). The calibrated visibilities were averaged into 8 MHz channels and imaged.…”

“…The sample in Shanahan et al (2019) includes additional polarized components of resolved sources, but excludes sources that, upon visual inspection, were considered to be bright diffuse sources. As part of our analysis we use polarized subregions of SNRs G46.8−0.3 and G39.2 −0.3, where each subregion we analyze is sized to be ∼16″, the approximate synthesized beam at 1.2 GHz (Shanahan et al 2022). For more information on the imaging and calibration, we refer to Section 2 in Shanahan et al (2022).…”

“…Polarimetry of an SNR reveals a blend of foreground and internal Faraday rotation. The study of Faraday rotation enables one to investigate internal turbulence in the expanding shell, plus a contribution from electron density and magnetic field fluctuations within the Galactic ISM along the line of sight between the SNR and the observer (Velusamy & Kundu 1975;Milne 1990;Simonetti 1992;Reich 2002;Wood et al 2008;Harvey-Smith et al 2010;Reynolds et al 2012;Farnes et al 2017;Bykov et al 2020;Shanahan et al 2022). However, separating internal turbulence of the SNR from turbulence in the ISM is challenging.…”

“…For our purposes, we use the method of Stokes QU fitting (Law et al 2011;O'Sullivan et al 2017). Shanahan et al (2022) presents polarimetry of SNRs G46.8 −0.3 and G39.2−0.3 in which variations of Faraday rotation and polarized intensity are observed on the scales of their beam (16″). They also observe complex Faraday rotation in these SNRs, which they conclude is evidence of Faraday depth structure internal to the SNRs.…”

“…In Section 2 we present a brief overview of the observations. For a more detailed description of the observations, calibration, and imaging we refer to Beuther et al (2016) and Shanahan et al (2022). In Section 3 we outline the methods used in our investigation of complex Faraday rotation and magnetic turbulence.…”