| Abstract
| - Abstract. Galactic stellar-population-synthesis models, chemical-enrichment models, and possibly gravitational microlensing indicate that about Ntot=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, NNS∼109, their higher masses, 〈M〉∼9 M⊙, and smaller space velocities, συ∼40 km s-1, result in Bondi-Hoyle accretion rates ∼4×103 times higher than for neutron stars. Given a total number of black holes Ntot=N9109 within the Milky Way, we estimate that ∼103N9 should accrete at M˙>1015 g 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(NNS/Ntot)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)∼104N9(F-15/ε-5)-1.2 isolated accreting black holes for a velocity dispersion of 40 km 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.
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