Peculiar radio-bright behaviour of the Galactic black hole transient 4U 1543-47 in the 2021-2023 outburst

TL;DR

This study observed the Galactic black hole transient 4U 1543-47 during its 2021-2023 outburst, revealing unusual radio-bright behavior and a steeper radio-X-ray correlation than typical, likely due to relativistic jet effects.

Contribution

First detailed radio and X-ray monitoring of 4U 1543-47 during its outburst, uncovering peculiar radio-brightness and a steeper correlation indicating relativistic jet variability.

Findings

Radio-X-ray correlation index of 0.82±0.09, steeper than typical 0.6.

Maximum radio brightness significantly exceeds most BH XRBs.

Evidence for relativistic effects with Lorentz factor between 1 and 2.

Abstract

Correlated behaviours between the radio emission and the X-ray emission in Galactic black hole X-ray binaries (BH XRBs) in the X-ray hard state are crucial to the understanding of disc-jet coupling of accreting black holes. The BH transient 4U 1543-47 went into outburst in 2021 following ~19 years of quiescence. We followed it up with ~weekly cadence with MeerKAT for about one year and a half until it faded into quiescence. Multi-epoch quasi-simultaneous MeerKAT and X-ray observations allowed us to trace the compact jet emission and its X-ray emission. In its hard spectral state across three orders of magnitude of X-ray luminosities above ~10$^{34}$ ergs/s, we found the correlation between radio and X-ray emission had a power-law index of 0.82$\pm$0.09, steeper than the canonical value of ~0.6 for BH XRBs. In addition, the radio vs. X-ray correlation shows a large range of the power-law normalization, with the maximum significantly larger than that obtained for most BH XRBs, indicating it can be particularly radio-bright and variable in the X-ray binary sample. The radio emission is unlikely diluted by discrete jet components. The observed peculiar radio-bright and variable behaviours provide the evidence for the relativistic effects of a variable Lorentz factor in the range between 1 and ~2 of the compact jet.