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

Ludlam, R. M. and Jaodand, A. D. and García, J. A. and Degenaar, N. and Tomsick, J. A. and Cackett, E. M. and Fabian, A. C. and Gandhi, P. and Buisson, D. J. K. and Shaw, A. W. and Chakrabarty, D. (2021) Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 1543–624. Astrophysical Journal, 911 (2). Art. No. 123. ISSN 0004-637X. doi:10.3847/1538-4357/abedb0.

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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_⊙.

Item Type:Article
Related URLs:
URLURL TypeDescription
Ludlam, R. M.0000-0002-8961-939X
Jaodand, A. D.0000-0002-3850-6651
García, J. A.0000-0003-3828-2448
Tomsick, J. A.0000-0001-5506-9855
Cackett, E. M.0000-0002-8294-9281
Fabian, A. C.0000-0002-9378-4072
Gandhi, P.0000-0003-3105-2615
Shaw, A. W.0000-0002-8808-520X
Chakrabarty, D.0000-0001-8804-8946
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).
Group:Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51440.001
Alexander von Humboldt FoundationUNSPECIFIED
NASA Einstein FellowshipUNSPECIFIED
Subject Keywords:Accretion; Low-mass x-ray binary stars
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Accretion (14); Low-mass x-ray binary stars (939)
Record Number:CaltechAUTHORS:20210429-144554967
Persistent URL:
Official Citation:R. M. Ludlam et al 2021 ApJ 911 123
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108894
Deposited By: George Porter
Deposited On:29 Apr 2021 22:08
Last Modified:16 Nov 2021 19:33

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