Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543–624
Author(s)
Ludlam, R. M.; Jaodand, A. D.; García, J. A.; Degenaar, N.; Tomsick, J. A.; Cackett, E. M.; Fabian, A. C.; Gandhi, P.; Buisson, D. J. K.; Shaw, A. W.; Chakrabarty, D.; ... Show more Show less
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<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>
Date issued
2021-04Department
MIT Kavli Institute for Astrophysics and Space ResearchJournal
Astrophysical Journal
Publisher
American Astronomical Society
Citation
Ludlam, RM, Jaodand, AD, García, JA, Degenaar, N, Tomsick, JA et al. 2021. "Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543–624." Astrophysical Journal, 911 (2).
Version: Final published version
ISSN
0004-637X
1538-4357