A 1.9M⨀ Neutron Neutron Star Candidate in a 2-Year Orbit

Kareem El-Badry*, Joshua D. Simon, Henrique Reggiani, Hans Walter Rix, David W. Latham, Allyson Bieryla, Lars A. Buchhave, Sahar Shahaf, Tsevi Mazeh, Sukanya Chakrabarti, Puragra Guhathakurta, Ilya V. Ilyin, Thomas M. Tauris

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


We report discovery and characterization of a main-sequence G star orbiting a dark object with mass 1.90±0.04 M⨀. The system was discovered via Gaia astrometry and has an orbital period of 731 days. We obtained multi-epoch RV follow-up over a period of 639 days, allowing us to refine the Gaia orbital solution and precisely constrain the masses of both components. The luminous star is a≳12 Gyrold, low-metallicity halo star near the main-sequence turnoff (Teff ≈ 6000 K; log g/[cm s−2]) ≈ 4.0; [Fe/H] ≈−1.25; M ≈ 0.79 M⨀) with a highly enhanced lithium abundance. The RV mass function sets a minimum companion mass for an edge-on orbit of M2>1.67 M⨀, well above the Chandrasekhar limit. The Gaia inclination constraint, i=68.7±1.4 deg, then implies a companion mass of M2=1.90±0.04 M⨀. The companion is most likely a massive neutron star: the only viable alternative is two massive white dwarfs in a close binary, but this scenario is disfavored on evolutionary grounds. The system’s low eccentricity (e=0.122±0.002) disfavors dynamical formation channels and implies that the neutron star likely formed with little mass loss (≲1 M⨀) and with a weak natal kick (vkick≲20 km s−1). Stronger kicks with more mass loss are not fully ruled out but would imply that a larger population of similar systems with higher eccentricities should exist. The current orbit is too small to have accommodated the neutron star progenitor as a red supergiant or super-AGB star. The simplest formation scenario–isolated binary evolution–requires the system to have survived unstable mass transfer and common envelope evolution with a donor-to-accretor mass ratio>10. The system, which we call Gaia NS1, is likely a progenitor of symbiotic X-ray binaries and long-period millisecond pulsars. Its discovery challenges binary evolution models and bodes well for Gaia’s census of compact objects in wide binaries.

Original languageEnglish
JournalOpen Journal of Astrophysics
Number of pages23
Publication statusAccepted/In press - 2024


  • Binaries: spectroscopic
  • Stars: evolution
  • Stars: neutron


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