A Sample of Neutron Star and Black Hole Binaries Detected through Gaia DR3 Astrometry

With its exquisite astrometric precision, the latest Gaia data release includes $\sim$$10^5$ astrometric binaries, each of which have measured orbital periods, eccentricities, and the Thiele-Innes orbital parameters. Using these and an estimate of the luminous stars' masses, we derive the companion stars' masses, from which we identify a sample of 24 binaries in long period orbits ($P_{\rm orb}\sim{\rm yrs}$) with a high probability of hosting a massive ($>$1.4 $M_{\odot}$), dark companion: a neutron star (NS) or black hole (BH). The luminous stars in these binaries tend to be F-, G-, and K-dwarfs with the notable exception of one hot subdwarf. Follow-up spectroscopy of eight of these stars shows no evidence for contamination by white dwarfs or other luminous stars. The dark companions in these binaries span a mass range of 1.35-2.7 $M_{\odot}$ and therefore likely includes both NSs and BHs without a significant mass gap in between. Furthermore, the masses of several of these objects are $\simeq$1.7 $M_{\odot}$, similar to the mass of at least one of the merging compact objects in GW190425. Given that these orbits are too wide for significant mass accretion to have occurred, this sample implies that some NSs are born heavy ($\gtrsim$1.5 $M_{\odot}$). Additionally, the low orbital velocities ($\lesssim$20 km s$^{-1}$) of these binaries requires that at least some heavy NSs receive low natal kicks, otherwise they would have been disrupted during core collapse. Although none will become gravitational wave sources within a Hubble time, these systems will be exceptionally useful for testing binary evolution theory.