Galactic stellar-population-synthesis models , chemical-enrichment models , and long-duration Bulge microlensing events indicate about N _ { tot } = 10 ^ { 8 } -10 ^ { 9 } 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 isolated black holes may be fewer in number than neutron stars , N _ { NS } \sim 10 ^ { 9 } , their higher masses , \langle M \rangle \sim 9 { M } _ { \odot } , and smaller space velocities , \sigma _ { v } \sim 40 km s ^ { -1 } result in Bondi-Hoyle accretion rates \sim 4 \times 10 ^ { 3 } times higher than for neutron stars .
We estimate that \sim 10 ^ { 4 } isolated black holes within the Milky Way should accrete at \dot { M } > 10 ^ { 15 } g s ^ { -1 } , comparable to accretion rates inferred for black-hole X-ray binaries given a total number of black holes N _ { tot } = N _ { 9 } 10 ^ { 9 } .
If black holes accrete at efficiencies only \sim 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 which can be detected with X-ray surveys as a function of efficiency , concluding that all-sky surveys at a depth of F = F _ { -15 } 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } dex ^ { -1 } can find N ( > F ) \sim 10 ^ { 4 } N _ { 9 } ( F _ { -15 } / \epsilon _ { -5 } ) ^ { -1.2 } isolated accreting black holes for a velocity dispersion of 40 km s ^ { -1 } and X-ray accretion efficiency of \epsilon = \epsilon _ { -5 } 10 ^ { -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 minimum mass can be derived for microlensing black-hole candidates if they are detected in the X-ray .