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Title:
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.
Affiliation:
AA(Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556.; Center for Particle Astrophysics, University of California, 301 Le Conte Hall, Berkeley, CA 94720.), AB(Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974.), AC(Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556.), AD(Departments of Astronomy and Physics, University of Washington, Stevens Way, Box 351580, Seattle, WA 98195.), AE(Center for Particle Astrophysics, University of California, 301 Le Conte Hall, Berkeley, CA 94720.; Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104-6396.; Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.), AF(Supercomputing Facility, Australian National University, Canberra, ACT 0200, Australia.), AG(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218.), AH(Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia.), AI(Department of Physics, University of the Free State, Bloemfontein 9300, South Africa.), AJ(Center for Particle Astrophysics, University of California, 301 Le Conte Hall, Berkeley, CA 94720.; Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.), AK(Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.), AL(Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556.), AM(Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia.), AN(Department of Astronomy and Astrophysics, University of California , 477 Clark Kerr Hall, Santa Cruz, CA 95064.), AO(Department of Physics, University of California at San Diego, 9500 Gillman Drive, La Jolla, CA 92039.), AP(Tate Laboratory of Physics, University of Minnesota, Minneapolis, MN 55455.), AQ(Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.), AR(Departments of Astronomy and Physics, University of Washington, Stevens Way, Box 351580, Seattle, WA 98195.), AS(Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104-6396.), AT(Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.), AU(Departamento de Astronomia, Pontificia Universidad Catolica de Chile, Casilla 104, Santiago 22, Chile.), AV(Lawrence Livermore National Laboratory, University of California, 7000 East Avenue, Livermore, CA 94550.; Department of Physics, University of California, 366 Le Conte Hall, Berkely, CA 94720.), AW(Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia.), AX(Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85741 Garching bei München, Germany.), AY(Departments of Astronomy and Physics, University of Washington, Stevens Way, Box 351580, Seattle, WA 98195.), AZ(European Southern Observatory, Karl Schwarzchild Strasse 2, D-85748 Garching bei München, Germany.), BA(Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556.), BB(Center for Particle Astrophysics, University of California, 301 Le Conte Hall, Berkeley, CA 94720.; Departments of Astronomy and Physics, University of Washington, Stevens Way, Box 351580, Seattle, WA 98195.), BC(Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK.), BD(Department of Astronomy and Astrophysics, University of California , 477 Clark Kerr Hall, Santa Cruz, CA 95064.), BE(Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada.)
Publication:
The Astrophysical Journal, Volume 579, Issue 2, pp. 639-659. (ApJ Homepage)
Publication Date:
11/2002
Origin:
UCP
ApJ Keywords:
Galaxy: Bulge, Cosmology: Gravitational Lensing
DOI:
10.1086/342225
Bibliographic Code:
2002ApJ...579..639B

Abstract

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 Msolar, 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/Msolar=6+10-3 and M/Msolar=6+7-3, respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 Lsolar, 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, Górski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well. Based in part on observations from NASA's Hubble Space Telescope.
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