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Published September 20, 2019 | Accepted Version + Published
Journal Article Open

Observations of the Ultra-compact X-Ray Binary 4U 1543-624 in Outburst with NICER, INTEGRAL, Swift, and ATCA


We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543−624 taken in August 2017 during an enhanced accretion episode. We obtained Neutron Star Interior Composition Explorer (NICER) monitoring of the source over a ~10 day period during which target-of-opportunity observations were also conducted with Swift, INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and the Australia Telescope Compact Array. Emission lines were measured in the NICER X-ray spectrum at ~0.64 keV and ~6.4 keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii (R_g = GM/c^2) at the beginning of the outburst to <8.7 R g at peak intensity. We do not detect the source in radio, but are able to place a 3σ upper limit on the flux density at 27 μJy beam^(−1). Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the L_r-L_x plane, suggesting a neutron star primary. This adds to the evidence that neutron stars (NSs) do not follow a single track in the L_r-L_x plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.

Additional Information

© 2019 The American Astronomical Society. Received 2019 June 18; revised 2019 July 26; accepted 2019 July 31; published 2019 September 18. This work was supported by NASA through the NICER mission and the Astrophysics Explorers Program, and made use of data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC). The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. R.M.L. acknowledges partial funding from a NASA Earth and Space Science Fellowship. E.M.C. gratefully acknowledges support through CAREER award number AST-1351222. M.F. acknowledges ASI financial/programmatic support via ASI-INAF agreement n. 2013-025.R1 and ASI-INAF N. 2017-14-H.0. J.A.G. acknowledges support from Chandra Theory grant TM8-19003X and from the Alexander von Humboldt Foundation. J.C.A. M.-J. is the recipient of an Australian Research Council Future Fellowship (FT 140101082), funded by the Australian Government.

Attached Files

Published - Ludlam_2019_ApJ_883_39.pdf

Accepted Version - 1908.00539.pdf


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August 19, 2023
October 18, 2023