A long-lived compact jet in the black hole X-ray binary candidate AT2019wey

AT2019wey is a transient discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in December of 2019. Follow-up optical, radio, and X-ray observations led to classify this source as a Galactic black hole X-ray binary. We carried out one-epoch 6.7-GHz European VLBI (Very Long Baseline Interferometry) Network (EVN) and two-epoch multiple-frequency (1.6, 4.5, 6.7 GHz) Very Long Baseline Array (VLBA) observations within a year after it was first discovered. These observations reveal a fading and flat-spectrum radio source with no discernible motion. These features suggest the detection of a compact jet. The source appears resolved at milliarcsec scales, and the source angular size vs. frequency trend is consistent with scatter broadening. This allows us to constrain the lower limit of the source distance to 6 kpc, if the scattering medium is in a Galactic spiral arm. For a source location larger than 3 kpc, the estimated upper limit of the peculiar velocity, relative to the local standard of rest, suggests the asymmetric natal kick could happen during the black hole formation stage.