No X-Rays or Radio from the Nearest Black Holes and Implications for Future Searches

Abstract

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 $\sim 1\,M_{\odot}$ stars in wide orbits ($a\approx$1.4 AU, 4.96 AU) around $\sim9\,M_{\odot}$ 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 $L_X < 10^{29.4}$ and $L_X < 10^{30.1}\,\rm erg\,s^{-1}$ and radio upper limits of $L_r < 10^{25.2}$ and $L_r < 10^{25.9}\,\rm erg\,s^{-1}$. 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 $\sim 10^4$ 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.