Gaia's promise to detect compact-object binaries: where we stand with the third data release

Abstract

With its third data release (DR3), Gaia begins unveiling dormant candidate compact object (CO) binaries with luminous companions (LC) as predicted by several past theoretical studies. To date, 3 black hole (BH), 21 neutron star (NS), and 3200 white dwarf (WD) candidates have been identified with LCs in detached orbits using astrometry. We adopt an observationally motivated sampling scheme for the star formation history of the Milky Way, and initial zero-age main-sequence binary properties, incorporate all relevant binary interaction processes during evolution to obtain a realistic present-day intrinsic population of CO--LC binaries. We apply Gaia's selection criteria to identify the CO-LC binaries detectable using the observational cuts applicable for DR3 as well as its end-of-mission (EOM). We find that under the DR3 selection cuts, our detectable population includes no BH--LCs, approximately 10-40 NS--LCs, and around ~4300 WD--LCs. Our predicted NS--LC population is in good agreement with the current DR3 census, both in its predicted yield and in the orbital and stellar properties, and we recover a close analogue of the Gaia NS1 candidate together with its detailed formation pathway. For WD--LCs, we find that a moderate natal kick of 5-15 km/s imparted at WD formation is required to match the observed orbital properties of WD-LC candidates in DR3. We further show that Gaia BH3-like binaries can form through standard isolated binary evolution without invoking any additional modelling assumptions, whereas reproducing Gaia BH1 and BH2 remains challenging within this framework. Looking ahead to the EOM, we predict detection of ~30-300 BH--LCs, ~1500-5000 NS-LCs, and ~10^5-10^6 WD-LC binaries, primarily due to the significantly longer observational baseline.