Document Type
Article
Publication Date
6-2024
Abstract
We present simultaneous Chandra X-ray Observatory and Hubble Space Telescope observations of three certain (X5, X7, W37) and two likely (X4, W17) quiescent neutron star low-mass X-ray binaries (qLMXBs) in the globular cluster 47 Tuc. We study these systems in the X-ray, optical, and near-ultraviolet (NUV) using the simultaneous data and additional non-contemporaneous HST data. We have discovered a blue and variable NUV counterpart to W17. We have not securely identified the eclipsing qLMXB W37 in the optical or NUV. Deeper high-resolution imaging is needed to further investigate the faint NUV excess near the centre of the W37 error circle. We suggest that a previously identified optical astrometric match to X7 is likely the true counterpart. The H α emission and the location of the counterpart in the colour–magnitude diagram, indicate that the secondary is probably a non-degenerate, H-rich star. This is consistent with previous results from fitting X7’s X-ray spectrum. In X4, the simultaneous X-ray and optical behaviour supports the earlier suggestion that the X-ray variability is driven by changes in accretion rate. The X-ray eclipses in X5 coincide with minima in the optical/NUV light curves. Comparison of the 47 Tuc qLMXBs with the cataclysmic variables (CVs) in the cluster confirms that overall the qLMXBs have larger X-ray to optical flux ratios. Based on their optical/NUV colours, we conclude that the accretion discs in the qLMXBs are less prominent than in CVs. This makes the ratio of X-ray flux to excess blue-optical flux a powerful discriminator between CVs and qLMXBs.
Recommended Citation
van den Berg, Maureen, L. E. Rivera Sandoval, Craig O. Heinke, Haldan N. Cohn, Phyllis M. Lugger, Jonathan E. Grindlay, Peter D. Edmonds, Jay Anderson, and Andrei Catuneanu. "Simultaneous Chandra and HST observations of the quiescent neutron star low-mass X-ray binaries in 47 Tucanae." Monthly Notices of the Royal Astronomical Society 531, no. 1 (2024): 1653-1670. https://doi.org/10.1093/mnras/stae1087
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Publication Title
Monthly Notices of the Royal Astronomical Society
DOI
https://doi.org/10.1093/mnras/stae1087
Comments
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.