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 <it>N</it><inf>tot</inf>=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, <it>N</it><inf>NS</inf>∼109, their higher masses, 〈<it>M</it>〉∼9 M<inf>⊙</inf>, and smaller space velocities, <it>σ</it><inf>υ</inf>∼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 <it>N</it><inf>tot</inf>=<it>N</it><inf>9</inf>109 within the Milky Way, we estimate that ∼103<it>N</it><inf>9</inf> should accrete at <it><ac>M</ac><ac>˙</ac></it>>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(<it>N</it><inf>NS</inf>/<it>N</it><inf>tot</inf>)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 <it>F</it>=<it>F</it><inf>-15</inf>10-15 erg cm-2 s-1 dex-1 can find <it>N</it>(><it>F</it>)∼104<it>N</it><inf>9</inf>(<it>F</it><inf>-15</inf>/ϵ<inf>-5</inf>)-1.2 isolated accreting black holes for a velocity dispersion of 40 km s−1 and an X-ray accretion efficiency of ϵ=ϵ<inf>-5</inf>10-5. Deeper surveys of the Galactic plane with <it>Chandra</it> or <it>XMM-Newton</it> 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.