Spectrum and polarization of the Galactic center radio transient ASKAP J173608.2-321635 from THOR-GC and VLITE

The radio transient ASKAP J173608. 2-321735, at the position (l, b) = (356. 0872, -0. 0390), was serendipitously observed by The HI/OH/Recombination Line Survey of the Galactic Center (THOR-GC) at three epochs in March 2020, April 2020 and February 2021. The source was detected only on 2020 April 11 with flux density 20. 6 +/- 1. 1 mJy at 1. 23 GHz and in-band spectral index alpha = -3. 1 +/- 0. 2. The commensal VLA Low-band Ionsophere and Transient Experiment (VLITE) simultaneously detected the source at 339 MHz with a flux density 122. 6 +/- 20. 4 mJy, indicating a spectral break below 1 GHz. The rotation measure in April 2020 was 63. 9 +/- 0. 3rad/m2, which almost triples the range of the variable rotation measure observed by Wang et al. (2021) to ~130 rad/m2. The polarization angle, corrected for Faraday rotation, was 97 +/- 6 degrees. The 1. 23 GHz linear polarization was 76. 7% +/- 3. 9% with wavelength-dependent depolarization indicating Faraday depth dispersion sigmaₚhi = 4. 8^+0. 5-₀. ₇ rad/m2. We find an upper limit to circular polarization |V|/I < 10. 1%. Interpretation of the data in terms of diffractive scattering of radio waves by a plasma near the source indicates electron density and line-of-sight magnetic field strength within a factor 3 of nₑ ~2 cm^-3 and Bₚar ~2 x 10⁵ microgauss. Combined with causality limits to the size of the source, these parameters are consistent with the low-frequency spectral break resulting from synchrotron self-absorption, not free-free absorption. A possible interpretation of the source is a highly supersonic neutron star interacting with a changing environment.