A 1.9 solar-mass neutron star candidate in a 2-year orbit

We report discovery and characterization of a main-sequence G star orbiting a dark object with mass 1.90±0.04M⊙. 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,Gyr-old, low-metallicity halo star near the main-sequence turnoff (,K; ; ; M≈0.79M⊙) with a highly enhanced lithium abundance. The RV mass function sets a minimum companion mass for an edge-on orbit of M2>1.67M⊙, well above the Chandrasekhar limit. The Gaia inclination constraint, i=68.7±1.4,deg, then implies a companion mass of M2=1.90±0.04M⊙. 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 ( ≲1M⊙) and with a weak natal kick (). 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.