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Title:
X-rays from isolated black holes in the Milky Way
Authors:
Agol, Eric; Kamionkowski, Marc
Affiliation:
AA(California Institute of Technology, Mail Code 130-33, Pasadena, CA 91125, USA), AB(California Institute of Technology, Mail Code 130-33, Pasadena, CA 91125, USA)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 334, Issue 3, pp. 553-562. (MNRAS Homepage)
Publication Date:
08/2002
Origin:
MNRAS
MNRAS Keywords:
accretion, accretion discs; black hole physics; Galaxy: stellar content; X-rays: ISM; X-rays: stars
Abstract Copyright:
(c) 2002 Blackwell Science Ltd
DOI:
10.1046/j.1365-8711.2002.05523.x
Bibliographic Code:
2002MNRAS.334..553A

Abstract

Galactic stellar-population-synthesis models, chemical-enrichment models, and possibly gravitational microlensing indicate that about Ntot=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, NNS~109, their higher masses, <M>~9Msolar, and smaller space velocities, σv~40kms-1, result in Bondi-Hoyle accretion rates ~4×103 times higher than for neutron stars. Given a total number of black holes Ntot=N9109 within the Milky Way, we estimate that ~103N9 should accrete at Mȯ>1015gs-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(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 that can be detected with X-ray surveys as a function of efficiency, concluding that all-sky surveys at a depth of F=F-1510-15ergcm-2s- 1dex-1 can find N(>F)~104N9(F-15/ɛ- 5)-1.2 isolated accreting black holes for a velocity dispersion of 40kms-1 and an X-ray accretion efficiency of ɛ=ɛ-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.

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