Spectrum and Polarization of the Galactic Center Radio Transient ASKAP J173608.2–321635 from THOR-GC and VLITE

• Published in: The Astrophysical journal 2024-07, Vol.970 (1), p.92
• Main Authors: Weatherhead, Kierra J., Stil, Jeroen M., Rugel, Michael, Peters, Wendy M., Anderson, Loren, Barnes, Ashley, Beuther, Henrik, Clarke, Tracy E., Dzib, Sergio A., Goldsmith, Paul, Menten, Karl M., Nyland, Kristina E., Sormani, Mattia C., Urquhart, James
• Format: Article
• Language: English
• Subjects: Absorption; Changing environments; Circular polarization; Depolarization; Electron density; Environmental changes; Faraday effect; Field strength; Flux density; Galactic radio sources; Interstellar magnetic fields; Linear polarization; Magnetic fields; Neutron stars; Polarization; Radio bursts; Radio transient sources; Radio wave scattering; Radio waves; Sky surveys
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad4f8c

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 σ ϕ = 4.8 − 0.7 + 0.5 rad m − 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 × 10 5 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.