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          Institute: MPI für Astrophysik     Collection: Cosmology     Display Documents

ID: 10993.0, MPI für Astrophysik / Cosmology
Gravitational microlensing events due to stellar-mass black holes
Authors:Bennett, D. P.; Becker, A. C.; Quinn, J. L.; Tomaney, A. B.; Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Calitz, J. J.; Cook, K. H.; Drake, A. J.; Fragile, P. C.; Freeman, K. C.; Geha, M.; Griest, K.; Johnson, B. R.; Keller, S. C.; Laws, C.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Nelson, C. A.; Peterson, B. A.; Popowski, P.; Pratt, M. R.; Quinn, P. J.; Rhie, S. H.; Stubbs, C. W.; Sutherland, W.; Vandehei, T.; Welch, D.
Date of Publication (YYYY-MM-DD):2002
Title of Journal:The Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:2
Start Page:639
End Page:659
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distances and masses of the lens objects based on a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are greater than 1 M-circle dot, which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long-timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/M-circle dot = 6(-3)(+10) and M/M-circle dot = 6(-3)(+7), respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 L-circle dot, so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (Sgr) dwarf galaxy behind the bulge, but we find that bulge sources are likely to dominate our microlensing parallax event sample. Future Hubble Space Telescope observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by the Space Interferometry Mission or the Keck or VLT interferometers, as explained by Delplancke, Gorski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well.
Free Keywords:Galaxy : bulge; gravitational lensing
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astrophysik
External Affiliations:Univ Notre Dame, Dept Phys, 225 Nieuwland Sci Hall, Notre Dame,; IN 46556 USA; Univ Notre Dame, Dept Phys, Notre Dame, IN 46556 USA; Univ Calif Berkeley, Ctr Particle Astrophys, Berkeley, CA 94720 USA; Lucent Technol, Bell Labs, Murray Hill, NJ 07974 USA; Univ Washington, Dept Astron, Seattle, WA 98195 USA; Univ Washington, Dept Phys, Seattle, WA 98195 USA; Univ Penn, Dept Phys & Astron, Philadelphia, PA 19104 USA; Lawrence Livermore Natl Lab, Livermore, CA 94550 USA; Australian Natl Univ, Supercomp Facil, Canberra, ACT 0200, Australia; Space Telescope Sci Inst, Baltimore, MD 21218 USA; Mt Stromlo & Siding Spring Observ, Res Sch Astron & Astrophys, Weston, ACT 2611, Australia; Univ Orange Free State, Dept Phys, ZA-9300 Bloemfontein, South Africa; Univ Calif Santa Cruz, Dept Astron & Astrophys, Santa Cruz, CA 95064 USA; Univ Calif San Diego, Dept Phys, La Jolla, CA 92039 USA; Univ Minnesota, Tate Lab Phys, Minneapolis, MN 55455 USA; Pontificia Univ Catolica Chile, Dept Astron, Santiago 22, Chile; Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA; Max Planck Inst Astrophys, D-85741 Garching, Germany; European So Observ, D-85748 Garching, Germany; Univ Oxford, Dept Phys, Oxford OX1 3RH, England; McMaster Univ, Dept Phys & Astron, Hamilton, ON L8S 4M1, Canada
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