Astrometry from the Gaia mission was recently used to discover the two
nearest known stellar-mass black holes (BHs), Gaia BH1 and Gaia BH2. Both
systems contain βΌ1Mββ stars in wide orbits (aβ1.4 AU,
4.96 AU) around βΌ9Mββ BHs. These objects are among the first
stellar-mass BHs not discovered via X-rays or gravitational waves. The
companion stars -- a solar-type main sequence star in Gaia BH1 and a
low-luminosity red giant in Gaia BH2 -- are well within their Roche lobes.
However, the BHs are still expected to accrete stellar winds, leading to
potentially detectable X-ray or radio emission. Here, we report observations of
both systems with the Chandra X-ray Observatory and radio observations with the
Very Large Array (for Gaia BH1) and MeerKAT (for Gaia BH2). We did not detect
either system, leading to X-ray upper limits of LXβ<1029.4 and LXβ<1030.1ergsβ1 and radio upper limits of Lrβ<1025.2 and
Lrβ<1025.9ergsβ1. For Gaia BH2, the non-detection implies
that the the accretion rate near the horizon is much lower than the Bondi rate,
consistent with recent models for hot accretion flows. We discuss implications
of these non-detections for broader BH searches, concluding that it is unlikely
that isolated BHs will be detected via ISM accretion in the near future. We
also calculate evolutionary models for the binaries' future evolution using
Modules for Experiments in Stellar Astrophysics (MESA). We find that Gaia BH1
will be X-ray bright for 5--50 Myr when the star is a red giant, including 5
Myr of stable Roche lobe overflow. Since no symbiotic BH X-ray binaries are
known, this implies either that fewer than βΌ104 Gaia BH1-like binaries
exist in the Milky Way, or that they are common but have evaded detection,
perhaps due to very long outburst recurrence timescales.Comment: Submitted to PAS