Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 1543-624

Abstract

We present the first joint NuSTAR and NICER observations of the ultra-compact X-ray binary (UCXB) 4U 1543$-$624 obtained in 2020 April. The source was at a luminosity of $L_{0.5-50\mathrm{keV}}=4.9(D/7\mathrm{kpc}{)}^2\times 10^{36}$ ergs s${}^{-1}$ and showed evidence of reflected emission in the form of an O VIII line, Fe K line, and Compton hump within the spectrum. We used a full reflection model, known as xillverCO, that is tailored for the atypical abundances found in UCXBs, to account for the reflected emission. We tested the emission radii of the O and Fe line components and conclude that they originate from a common disk radius in the innermost region of the accretion disk ($R_{\mathrm{in}}\le 1.07R_{\mathrm{ISCO}}$). Assuming that the compact accretor is a neutron star (NS) and the position of the inner disk is the Alfvén radius, we placed an upper limit on the magnetic field strength to be $B\le 0.7(D/7\mathrm{kpc})\times 10^8$ G at the poles. Given the lack of pulsations detected and position of $R_{\mathrm{in}}$, it was likely that a boundary layer region had formed between the NS surface and inner edge of the accretion disk with an extent of 1.2 km. This implies a maximum radius of the neutron star accretor of $R_{\mathrm{NS}}\le 12.1$ km when assuming a canonical NS mass of 1.4 $M_{\odot }$.

Accepted for publication in ApJ, 10 pages, 6 figures, 1 table