A 1.9\,M_{\odot} neutron star candidate in a 2-year orbit

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We report discovery and characterization of a main-sequence G star orbiting a dark object with mass 1.90\pm 0.04\,M_{\odot}. 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 600 days, allowing us to refine the Gaia orbital solution and precisely constrain the masses of both components. The luminous star is a \gtrsim 12 Gyr-old, low-metallicity halo star near the main-sequence turnoff (T_{\rm eff} \approx 6000 K; \log\left(g/\left[{\rm cm\,s^{-2}}\right]\right)\approx 4.0; \rm [Fe/H]\approx-1.25; M\approx0.79\,M_{\odot}) with a highly enhanced lithium abundance. The RV mass function sets a minimum companion mass for an edge-on orbit of M_2 > 1.67\,M_{\odot}, well above the Chandrasekhar limit. The Gaia inclination constraint, i=68.8\pm 1.4 deg, then implies a companion mass of M_2= 1.90\pm 0.04\,M_{\odot}. 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\pm 0.003) disfavors dynamical formation channels and implies that the neutron star formed with very little mass loss (\lesssim 1\,M_{\odot}) and with a weak natal kick (v_{\rm kick}\lesssim 10\,\rm 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 stable mass transfer or 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 binarie