2002MNRAS.334..553A

Galactic stellar-population-synthesis models, chemical-enrichment models, and possibly gravitational microlensing indicate that about N_tot=10⁸-10⁹ stellar-mass black holes reside in our Galaxy. We study X-ray emission from accretion from the interstellar medium on to isolated black holes. Although black holes may be fewer in number than neutron stars, N_NS∼10⁹, their higher masses, <M >∼9M_☉, and smaller space velocities, σ_v∼40km.s^–1, result in Bondi-Hoyle accretion rates ∼4x10³ times higher than for neutron stars. Given a total number of black holes N_tot=N₉10⁹ within the Milky Way, we estimate that ∼10³N₉should accrete at M{dot}>10¹⁵g.s^–1, comparable to accretion rates inferred for black hole X-ray binaries. If black holes accrete at the Bondi-Hoyle rate with efficiencies only ∼10^–4(N_NS/N_tot)^0.8 of the neutron-star accretion efficiency, a comparable number of each may be detectable. We make predictions for the number of isolated accreting black holes in our Galaxy that can be detected with X-ray surveys as a function of efficiency, concluding that all-sky surveys at a depth of F =F_–1510^–15 erg.cm^–2.s^–1.dex^–1 can find N(>F)∼10⁴ N₉(F_–15/ε_–5)^–1.2 isolated accreting black holes for a velocity dispersion of 40km.s^–1 and an X-ray accretion efficiency of ε=ε_–510^–5. Deeper surveys of the Galactic plane with Chandra or XMM-Newton may find tens of these objects per year, depending on the efficiency. We argue that a mass estimate can be derived for microlensing black hole candidates with an X-ray detection.