2021ApJ...911..123L

We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary (UCXB) 4U 1543-624 obtained in 2020 April. The source was at a luminosity of L_0.5-50 keV_ = 4.9(D/7 kpc)² x 10³⁶ erg 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_in <= 1.07 R_ISCO). Assuming that the compact accretor is a neutron star (NS) and the position of the inner disk is the Alfven radius, we placed an upper limit on the magnetic field strength to be B <= 0.7(D/7 kpc) x 10⁸ G at the poles. Given the lack of pulsations detected and position of R_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_NS <= 12.1 km when assuming a canonical NS mass of 1.4 M_☉.