X-rays from isolated black holes in the Milky Way

Galactic stellar-population-synthesis models, chemical-enrichment models, and possibly gravitational microlensing indicate that about N-to =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 black holes may be fewer in number than neutron stars, N-NS similar to10(9) , their higher masses, <M>similar to9 M-circle dot, and smaller space velocities, sigma (v)similar to40 km s(-1) , result in Bondi-Hoyle accretion rates similar to4x10(3) times higher than for neutron stars. Given a total number of black holes N-tot =N(9)10(9) within the Milky Way, we estimate that similar to10(3)N(9) should accrete at (M)over dot >10(15) 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 similar to10(-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(-15)10(-15) erg cm(-2) s(-1) dex(-1) can find N (>F )similar to10(4) N-9(F-15/epsilon(-5) )(-1.2) isolated accreting black holes for a velocity dispersion of 40 km s(-1) and an 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 mass estimate can be derived for microlensing black hole candidates with an X-ray detection.