Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543–624

Abstract

<jats:title>Abstract</jats:title> <jats:p>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 <jats:italic>L</jats:italic> <jats:sub>0.5−50 keV</jats:sub> = 4.9(<jats:italic>D</jats:italic>/7 kpc)<jats:sup>2</jats:sup> × 10<jats:sup>36</jats:sup> erg s<jats:sup>−1</jats:sup> and showed evidence of reflected emission in the form of an O <jats:sc>viii</jats:sc> line, Fe K line, and Compton hump within the spectrum. We used a full reflection model, known as <jats:sc>xillverCO</jats:sc>, 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 (<jats:italic>R</jats:italic> <jats:sub>in</jats:sub> ≤ 1.07 <jats:italic>R</jats:italic> <jats:sub>ISCO</jats:sub>). 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 <jats:italic>B</jats:italic> ≤ 0.7(<jats:italic>D</jats:italic>/7 kpc) × 10<jats:sup>8</jats:sup> G at the poles. Given the lack of pulsations detected and position of <jats:italic>R</jats:italic> <jats:sub>in</jats:sub>, 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 <jats:italic>R</jats:italic> <jats:sub>NS</jats:sub> ≤ 12.1 km when assuming a canonical NS mass of 1.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>.</jats:p>