2024ApJ...973...75C

Gaia BH3 is the most massive known stellar-origin black hole in the Milky Way, with a mass M_• ≈ 33 M_⊙. Detected from Gaia's astrometry, this black hole is in the mass range of those observed via gravitational waves, whose nature is still highly debated. Hosted in a binary system with a companion giant star that is too far away for Roche-lobe mass transfer, this black hole could nonetheless accrete at low levels due to wind-driven mass loss from its companion star, thus accreting in advection-dominated accretion flow, or ADAF, mode. Using stellar wind models, we constrain its Eddington ratio in the range 10^–9 < f_Edd < 10^–7, corresponding to radiative efficiencies 5 × 10^–5 < ϵ < 10^–3, compatible with radiatively inefficient accretion modes. Chandra ACIS-S observed this object and obtained the most sensitive upper bound of its [2-10] keV flux: F_X < 3.25 × 10^–15 erg s^–1 cm^–2 at 90% confidence level corresponding to L_[2–10] < 2.10 × 10²⁹ erg s^–1. Using ADAF emission models, we constrained its accretion rate to f_Edd < 4.91 × 10^–7 at the apastron, in agreement with our theoretical estimate. At the periastron, we expect fluxes ∼50 times larger. Because of the inferred low rates, accretion did not significantly contribute to black hole growth over the system's lifetime. Detecting the electromagnetic emission from Gaia BH3 will be fundamental to informing stellar wind and accretion disk models.