A 1. 9\, M_ 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. 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 Gyr-old, low-metallicity halo star near the main-sequence turnoff (T ₄₅₅ 6000 K; (g/ cm\, s^-2) 4. 0; Fe/H-1. 25; M0. 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. 900. 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 (v ₊₈₂₊ 20\, km\, s^-1). 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.