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

TL;DR

This study reports the detection of a long-lived, compact jet in the black hole X-ray binary candidate AT2019wey, using VLBI observations that reveal a stable, flat-spectrum radio source with no apparent motion, constraining its distance and natal kick.

Contribution

First VLBI observations of AT2019wey revealing a persistent, compact jet and constraining its distance and velocity, providing insights into black hole formation and jet properties.

Findings

Detected a stable, flat-spectrum radio source indicating a compact jet.

Constrained the source distance to at least 6 kpc based on scatter broadening.

Estimated the upper limit of the peculiar velocity, suggesting a natal kick during black hole formation.

Abstract

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 classification of 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 its discovery. 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 milliarcsecond scales, and the source angular size versus 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 may have occurred during the black hole formation stage.