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Probable dormant neutron star in a short-period binary system

T. Mazeh, S. Faigler, D. Bashi, S. Shahaf, N. Davidson, M. Green, R. Gomel, D. Maoz, A. Sussholz, S. Dong, H. Zhang, J. Liu, S. Wang, A. Luo, Z. Zheng, N. Hallakoun, V. Perdelwitz, D. W. Latham, I. Ribas, D. Baroch, J. C. Morales, E. Nagel, N. C. Santos, D. Ciardi, J. L. Christiansen, M. B. Lund, J. N. Winn

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.

binaries: close, binaries: spectroscopic, stars: neutron

Monthly Notices of the Royal Astronomical Society
Volume 517, Issue 3, Page 4005
2022 December

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia