X-rays from isolated black holes in the Milky Way
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 M⊙>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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2002
- DOI:
- 10.1046/j.1365-8711.2002.05523.x
- arXiv:
- arXiv:astro-ph/0109539
- Bibcode:
- 2002MNRAS.334..553A
- Keywords:
-
- accretion;
- accretion discs;
- black hole physics;
- Galaxy: stellar content;
- X-rays: ISM;
- X-rays: stars;
- Astrophysics
- E-Print:
- 13 pages, 7 figures, submitted to MNRAS