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Published August 11, 2002 | Accepted Version + Published
Journal Article Open

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_(tot) = 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, N_(ɴꜱ)∼10⁹, their higher masses, 〈M〉∼9 M_⊙, and smaller space velocities, σ_υ∼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 N_(tot) = N₉10⁹ within the Milky Way, we estimate that ∼10³N₉ should accrete at Ṁ > 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(N_(ɴꜱ)/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₋₁₅10⁻¹⁵ erg cm⁻² s⁻¹ dex⁻¹ can find N(> F)∼10⁴N⁹(F₋₁₅/ε₋₅)^(1.2) isolated accreting black holes for a velocity dispersion of 40 km s⁻¹ and an X-ray accretion efficiency of ε = ε-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 mass estimate can be derived for microlensing black hole candidates with an X-ray detection.

Additional Information

© 2002 RAS. Received: 22 February 2002. Accepted: 22 March 2002. Published: 11 August 2002. We thank D. Bennett, L. Bildsten, O. Blaes, R. Blandford, J. Carpenter, D. Chernoff, G. Dubus, A. Esin, C. Fryer, J. Grindlay, T. Kallman, L. Koopmans, J. Krolik, Y. Lithwick, A. Melatos, S. Phinney, M. Rees, R. Rutledge, N. Scoville and K. Sheth for useful conversations and ideas which greatly improved this work. This work was supported in part by NSF AST-0096023, NASA NAG5-8506, and DoE DE-FG03-92-ER40701. Support for the work done by EA was provided by the National Aeronautics and Space Administration through Chandra Postdoctoral Fellowship Award Number PF0-10013 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-39073.

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