Probable dormant neutron star in a short-period binary system. Issue 3 (7th October 2022)

• Record Type: Journal Article
• Authors: Mazeh, Tsevi; Faigler, Simchon; Bashi, Dolev; Shahaf, Sahar; Davidson, Niv; Green, Matthew; Gomel, Roy; Maoz, Dan; Sussholz, Amitay; Dong, Subo; Zhang, Haotong; Liu, Jifeng; Wang, Song; Luo, Ali; Zheng, Zheng; Hallakoun, Na'ama; Perdelwitz, Volker; Latham, David W; Ribas, Ignasi; Baroch, David; Morales, Juan Carlos; Nagel, Evangelos; Santos, Nuno C; Ciardi, David R; Christiansen, Jessie L; Lund, Michael B; Winn, Joshua N
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 517:Issue 3(2022)
• Page Start: 4005
• Page End: 4021
• Publication Date: 2022-10-07
• Subjects: binaries: close -- binaries: spectroscopic -- stars: neutron; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2853
• Languages: English
• ISSNs: 0035-8711

Abstract

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–$2.1\, \mathrm{M}_{\odot }$ at 3σ confidence, with the median of the mass distribution at $1.4\, \mathrm{M}_{\odot }$, 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.