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Published April 20, 2021 | Published
Journal Article Open

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


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 × 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 Alfvén radius, we placed an upper limit on the magnetic field strength to be B ≤ 0.7(D/7 kpc) × 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_⊙.

Additional Information

© 2021. The American Astronomical Society. Received 2020 December 7; revised 2021 March 8; accepted 2021 March 9; published 2021 April 26. Support for this work was provided by NASA through the NASA Hubble Fellowship grant #HST-HF2-51440.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Additional support was provided from NASA under the NICER Guest Observer grant 80NSSC21K0121. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA). J.A.G. acknowledges support from NASA grant 80NSSC20K1238 and from the Alexander von Humboldt Foundation. E.M.C. gratefully acknowledges support through NSF CAREER award number AST-1351222. J.A.T. acknowledges partial support from NASA under NICER Guest Observer grant 80NSSC19K1445. D.J.K.B. acknowledges funding from the Royal Society. Facilities: ADS - , HEASARC - , NICER - , NuSTAR. - Software: HEAsoft (v6.27.2, HEASARC 2014), nustardas (v1.9.2), nicerdas (2020-04-23_V007a), xspec (v12.11.0, Arnaud 1996), Stingray (Huppenkothen et al. 2019).

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August 22, 2023
October 23, 2023