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We report on X-ray and radio observations of the ultra-compact X-ray binary4U 1543-624 taken in August 2017 during an enhanced accretion episode. Weobtained NICER monitoring of the source over a $\sim10$ day period during whichtarget-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. Bymodeling these line components, we are able to track changes in the accretiondisk throughout this period. The innermost accretion flow appears to moveinwards from hundreds of gravitational radii ($R_{g}=GM/c^{2}$) at thebeginning of the outburst to $<8.7$ $R_{g}$ at peak intensity. We do not detectthe source in radio, but are able to place a $3\sigma$ upper limit on the fluxdensity at $27$ $\mu$Jy beam$^{-1}$. Comparing the radio and X-rayluminosities, we find that the source lies significantly away from the rangetypical of black holes in the ${L}_{{r}}$-${L}_{{x}}$ plane, suggesting aneutron star (NS) primary. This adds to the evidence that NSs do not follow asingle track in the ${L}_{{r}}$-${L}_{{x}}$ plane, limiting its use indistinguishing between different classes of NSs based on radio and X-rayobservations alone.

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