X-Rays from Isolated Black Holes in the Milky Way

Galactic stellar-population-synthesis models, chemical-enrichment models, and longduration Bulge microlensing events indicate about Ntot = 10 − 10 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, NNS ∼ 10, their higher masses, 〈M〉 ∼ 9M , and smaller space velocities, σv ∼ 40 km s−1 result in Bondi-Hoyle accretion rates ∼ 4× 10 times higher than for neutron stars. We estimate that ∼ 10 isolated black holes within the Milky Way should accrete at Ṁ > 10 g s−1, comparable to accretion rates inferred for blackhole X-ray binaries given a total number of black holes Ntot = N910. If black holes accrete at 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 which 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 ) ∼ 10N9(F−15/ −5)−1.2 isolated accreting black holes for a velocity dispersion of 40 km s−1 and X-ray accretion efficiency of = −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 minimum mass can be derived for microlensing black-hole candidates if they are detected in the X-ray.