Kierra Weatherhead

University of Calgary

Kierra Weatherhead is a first-year PhD student at the University of Calgary, working with Dr. Jeroen Stil. Her research focuses on studying the Galactic magnetic field near the Galactic center, home to the Milky Way’s supermassive black hole, Sgr A*. Kierra first got involved with radio astronomy during her undergraduate research and has since continued to study the Galactic magnetic field throughout her Master’s and PhD programs. She is currently continuing her work performing the polarization calibration of the HI/OH/Recombination Line Survey of the Galactic Center (THOR-GC).

During her Master’s research, an unusually highly polarized source was detected in THOR-GC. This radio transient, ASKAP J173608.2—321635, was first discovered in January 2020 by Wang et al. (2021). The polarization of this source in THOR-GC were consistent with previous findings, and the THOR-GC observation provided the first measurement of its intrinsic polarization angle. Combined simultaneous data from THOR-GC and the VLA Low-band Ionosphere and Transient Experiment revealed a low- frequency spectral break, which may be due to synchrotron self-absorption. The THOR-GC data also indicated that the rotation measure variability of ASKAP J173608.2—321635 is approximately three times larger than previously observed.

By applying a model for depolarization and induced circular polarization by diffractive scattering (Beniamini et al. 2022), the observed depolarization and circular polarization indicate that this source exists in a highly magnetized region. This environment is consistent with the observed rotation measure variability of ASKAP J173608.2—321635, assuming that it is traveling at a velocity of 1000 km/s through the interstellar medium, similar to the “Cannonball pulsar”.

Stokes I flux density of ASKAP J173608.2−321635 from THOR-GC and VLITE observations on 2020 April 11. A power law relation (Sν∼να) is fitted (solid) to the THOR-GC data with α=−3.1 ± 0.2. A power law relation with free-free absorption is fitted (dashed) to the combined THOR-GC and VLITE data with α=−3.6 ± 0.3 and τ=0.4 ± 0.06 at 1 GHz.