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

The radio transient ASKAP J173608.2–321635, at the position (ℓ, b) = (356.°0872, −0.°0390), was serendipitously observed by The H i/OH/Recombination line survey of the Galactic center at three epochs in 2020 March, 2020 April, and 2021 February. The source was detected only on 2020 April 11 with a flux density of 20.6 ± 1.1 mJy at 1.23 GHz and in-band spectral index of α = −3.1 ± 0.2. The commensal Very Large Array Low-band Ionsophere and Transient Experiment simultaneously detected the source at 339 MHz with a flux density of 122.6 ± 20.4 mJy, indicating a spectral break below 1 GHz. The rotation measure (RM) in 2020 April was 63.9 ± 0.3 rad m−2, which almost triples the range of the variable RM observed by Wang et al. to ∼130 rad m−2. The polarization angle, corrected for Faraday rotation, was 97° ± 6°. The 1.23 GHz linear polarization was 76.7% ± 3.9% with wavelength-dependent depolarization, indicating a Faraday depth dispersion of σϕ=4.8−0.7+0.5radm−2 . We find an upper limit to the circular polarization of ∣V∣/I < 10.1%. Interpretation of the data in terms of diffractive scattering of radio waves by a plasma near the source indicates an electron density and a line-of-sight magnetic field strength within a factor of 3 of n e ∼ 2 cm−3 and B ∥ ∼ 2 × 105 μ G. 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.