Direct detection of a microlens in the Milky Way

C. Alcock, R. A. Allsman, D. R. Alves, T. S. Axelrod, A. C. Becker, D. P. Bennett, K. H. Cook, A. J. Drake, K. C. Freeman, M. Geha, K. Griest, S. C. Keller, M. J. Lehner, S. L. Marshall, D. Minniti, C. A. Nelson (), B. A. Peterson, P. Popowski, M. R. Pratt, P. J. Quinn, C. W. Stubbs, W. Sutherland, A. B. Tomaney, T. Vandehei and D. Welch
Additional contact information
C. Alcock: Lawrence Livermore National Laboratory
R. A. Allsman: Supercomputing Facility, Australian National University
D. R. Alves: Space Telescope Science Institute
T. S. Axelrod: Research School of Astronomy and Astrophysics, Mount Stromlo Observatory
A. C. Becker: Bell Laboratories, Lucent Technologies
D. P. Bennett: Center for Particle Astrophysics, University of California
K. H. Cook: Lawrence Livermore National Laboratory
A. J. Drake: Lawrence Livermore National Laboratory
K. C. Freeman: Research School of Astronomy and Astrophysics, Mount Stromlo Observatory
M. Geha: University of California
K. Griest: University of California
S. C. Keller: Lawrence Livermore National Laboratory
M. J. Lehner: University of Pennsylvania
S. L. Marshall: Lawrence Livermore National Laboratory
D. Minniti: Depto. de Astronomia, P. Universidad Catolica
C. A. Nelson: Lawrence Livermore National Laboratory
B. A. Peterson: Research School of Astronomy and Astrophysics, Mount Stromlo Observatory
P. Popowski: Lawrence Livermore National Laboratory
M. R. Pratt: University of Washington
P. J. Quinn: European Southern Observatory
C. W. Stubbs: Center for Particle Astrophysics, University of California
W. Sutherland: University of Oxford
A. B. Tomaney: University of Washington
T. Vandehei: University of California
D. Welch: McMaster University

Nature, 2001, vol. 414, issue 6864, 617-619

Abstract: Abstract The nature of dark matter remains mysterious, with luminous material accounting for at most ∼25 per cent of the baryons in the Universe1,2. We accordingly undertook a survey looking for the microlensing of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.

Date: 2001
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DOI: 10.1038/414617a

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