Probable dormant neutron star in a short-period binary system

Creators: Mazeh, Tsevi¹; Faigler, Simchon¹; Bashi, Dolev¹; Shahaf, Sahar^{1, 2}; Davidson, Niv¹; Green, Matthew¹; Gomel, Roy¹; Maoz, Dan¹; Sussholz, Amitay¹; Dong, Subo³; Zhang, Haotong⁴; Liu, Jifeng⁴; Wang, Song⁴; Luo, Ali⁴; Zheng, Zheng⁵; Hallakoun, Na'ama²; Perdelwitz, Volker^{6, 7}; Latham, David W.⁸; Ribas, Ignasi^{9, 10}; Baroch, David^{9, 10}; Morales, Juan Carlos^{9, 10}; Nagel, Evangelos^{7, 11}; Santos, Nuno C.¹²; Ciardi, David R.^{13, 14, 15}; Christiansen, Jessie L.^{13, 14, 15}; Lund, Michael B.^{13, 14, 15}; Winn, Joshua N.¹⁶

1. Tel Aviv University
2. Weizmann Institute of Science
3. Peking University
4. National Astronomical Observatories
5. University of Utah
6. Ariel University
7. Universität Hamburg
8. Harvard-Smithsonian Center for Astrophysics
9. Institute of Space Sciences
10. Institut d'Estudis Espacials de Catalunya
11. Thüringer Landessternwarte Tautenburg
12. University of Porto
13. NASA Exoplanet Science Institute
14. Infrared Processing and Analysis Center
15. California Institute of Technology
16. Princeton University

Abstract

We have identified 2XMM J125556.57+565846.4, at a distance of 600 pc, as a binary system consisting of a normal star and a probable dormant neutron star. Optical spectra exhibit a slightly evolved F-type single star, displaying periodic Doppler shifts with a 2.76-d Keplerian circular orbit, with no indication of light from a secondary component. Optical and UV photometry reveal ellipsoidal modulation with half the orbital period, due to the tidal deformation of the F-star. The mass of the unseen companion is constrained to the range of 1.1–2.1M⊙ at 3σ confidence, with the median of the mass distribution at 1.4M⊙⁠, the typical mass of known neutron stars. A main-sequence star cannot masquerade as the dark companion. The distribution of possible companion masses still allows for the possibility of a very massive white dwarf. The companion itself could also be a close pair consisting of a white dwarf and an M star, or two white dwarfs, although the binary evolution that would lead to such a close triple system is unlikely. Similar ambiguities regarding the certain identification of a dormant neutron star are bound to affect most future discoveries of this type of non-interacting system. If the system indeed contains a dormant neutron star, it will become, in the future, a bright X-ray source and afterwards might even host a millisecond pulsar.

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This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model).

Acknowledgement

We thank the anonymous referee for thoughtful comments and suggestions that improved the original manuscript, and H-W Rix for discussion of the beaming modulation. This research was supported by Grant No. 2016069 of the United States-Israel Binational Science Foundation (BSF) to TM, Grant No. I-1498-303.7/2019 of the German-Israeli Foundation for Scientific Research and Development (,GIF) to TM, and DM, a grant from the European Research Council (ERC) under the European Union’s FP7 Programme, Grant No. 833031 to DM, the National Key R&D Program of China (No. 2019YFA0405100), the National Natural Science Foundation of China (Grant No. 12133005) and the XPLORER PRIZE to SD. The research of NH and SS is supported by a Benoziyo prize postdoctoral fellowship.

This paper uses data from the LAMOST survey: Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.

This work has also made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/ gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

Software References

This research made use of exoplanet (Foreman-Mackey et al. 2021) and its dependencies (Astropy Collaboration 2013, 2018; Salvatier et al. 2016; Team et al. 2016; Luger et al. 2019; Agol, Luger & Foreman-Mackey 2020).

Data Availability

Data used in this study are available upon request from the corresponding author.

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