Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543–624
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Ludlam_2021_ApJ_911_123.pdf
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Published version
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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.
Date Issued
April 2021
Journal
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
Abstract
Abstract
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 ⊙.
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 ⊙.
MIT Department
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
https://doi.org/10.3847/1538-4357/abedb0
