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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. [ABSTRACT FROM AUTHOR]

A black hole (BH) traveling through a uniform, gaseous medium is described by Bondi–Hoyle–Lyttleton (BHL) accretion. If the medium is magnetized, then the black hole can produce relativistic outflows. We performed the first 3D, general-relativistic magnetohydrodynamic simulations of BHL accretion onto rapidly rotating black holes using the H-AMR code, where we mainly varied the strength of a background magnetic field that threads the medium. We found that the ensuing accretion continuously drags the magnetic flux to the BH, which accumulates near the event horizon until it becomes dynamically important. Depending on the strength of the background magnetic field, the BHs can sometimes launch relativistic jets with high enough power to drill out of the inner accretion flow, become bent by the headwind, and escape to large distances. For stronger background magnetic fields, the jets are continuously powered, while at weaker field strengths they are intermittent, turning on and off depending on the fluctuating gas and magnetic flux distributions near the event horizon. We find that our jets reach extremely high efficiencies of ∼100%–300%, even in the absence of an accretion disk. We also calculated the drag forces exerted by the gas onto to the BH and found that the presence of magnetic fields causes the drag forces to be much less efficient than in unmagnetized BHL accretion. They can even sometimes become negative, accelerating the BH rather than slowing it down. Our results extend classical BHL accretion to rotating BHs moving through magnetized media, and demonstrate that accretion and drag are significantly altered in this environment. [ABSTRACT FROM AUTHOR]

Letter to the editor