X-Rays from Isolated Black Holes in the Milky Way

Galactic stellar-population-synthesis models, chemical-enrichment models, and long-duration Bulge microlensing events indicate about N_tot=(0.1-1) billion 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 ~ 1 billion, their higher masses, <M>~9 M_sun, and smaller space velocities, v ~ 40 km/s, result in Bondi-Hoyle accretion rates ~4.E3 times higher than for neutron stars. We estimate that ~1.E4 N_9 isolated black holes within the Milky Way should accrete at > 1.E15 g/s, comparable to accretion rates inferred for black-hole X-ray binaries given a total number of black holes N_tot= N_9 1.E9. If black holes accrete at efficiencies only ~1.E-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 1.E-15 erg/cm^2/s/dex can find N(>F)~ 1.E4 N_9 (e5/f)^1.2 isolated accreting black holes for a velocity dispersion of 40 km/s and X-ray accretion efficiency = e5 1.E-5. Deeper surveys of the Galactic plane with Chandra and 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.