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
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.
Keywords
binaries: close, binaries: spectroscopic, stars: neutron
Monthly Notices of the Royal Astronomical Society
Volume 517, Issue 3, Page 4005
2022 December
