# Observations of the Ultra-compact X-ray Binary 4U 1543-624 in Outburst with NICER, INTEGRAL, Swift, and ATCA

## TL;DR

This study presents multi-wavelength observations of the ultra-compact X-ray binary 4U 1543-624 during an outburst, revealing accretion disk dynamics and constraining the nature of its compact object.

## Contribution

It provides detailed X-ray and radio observational data during an outburst, tracking accretion disk evolution and identifying the neutron star nature of the primary.

## Key findings

- Inner accretion disk moved inward during outburst
- No radio detection, upper limit set at 27 μJy
- Source's radio and X-ray luminosities suggest a neutron star primary

## Abstract

We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543-624 taken in August 2017 during an enhanced accretion episode. We obtained NICER monitoring of the source over a $\sim10$ day period during which target-of-opportunity observations were also conducted with Swift, INTEGRAL, and ATCA. Emission lines were measured in the NICER X-ray spectrum at $\sim0.64$ keV and $\sim6.4$ keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii ($R_{g}=GM/c^{2}$) at the beginning of the outburst to $<8.7$ $R_{g}$ at peak intensity. We do not detect the source in radio, but are able to place a $3\sigma$ upper limit on the flux density at $27$ $\mu$Jy beam$^{-1}$. Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the ${L}_{{r}}$-${L}_{{x}}$ plane, suggesting a neutron star (NS) primary. This adds to the evidence that NSs do not follow a single track in the ${L}_{{r}}$-${L}_{{x}}$ plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.