VLBA Discovery of a Resolved Source in the Candidate Black Hole X-Ray Binary AT2019wey

AT2019wey is a Galactic low-mass X-ray binary with a candidate black hole accretor first discovered as an optical transient by ATLAS in 2019 December. It was then associated with an X-ray source discovered by SRG/eROSITA and SRG/ART-XC instruments in 2020 March. After a brightening in X-rays in 2020 August, VLA observations of the source revealed an optically thin spectrum that subsequently shifted to optically thick, as the source continued to brighten in the radio. This motivated us to observe AT2019wey with the VLBA. We found a resolved source that we interpret to be a steady compact jet, a feature associated with black hole X-ray binary systems in hard X-ray spectral states. The jet power is comparable to the accretion-disk X-ray luminosity. Here, we summarize the results from these observations.