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Letters to Nature
Nature 365, 621 - 623 (14 October 1993); doi:10.1038/365621a0

Possible gravitational microlensing of a star in the Large Magellanic Cloud

C. Alcock*, C. W. Akerlof, R. A. Allsman*, T. S. Axelrod*, D. P. Bennett*, S. Chan, K. H. Cook*, K. C. Freeman, K. Griest, S. L. Marshall§, H-S. Park*, S. Perlmutter, B. A. Peterson, M. R. Pratt§, P. J. Quinn, A. W. Rodgers, C. W. Stubbs§ & W. Sutherland

*Lawrence Livermore National Laboratory, Livermore, California 94550, USA
Center for Particle Astrophysics, University of California, Berkeley, California 94720, USA
Mt Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611, Australia
§Department of Physics, University of California, Santa Barbara, California 93106, USA
Department of Physics, University of California, San Diego, California 92039, USA
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA

THERE is now abundant evidence for the presence of large quantities of unseen matter surrounding normal galaxies, including our own1,2. The nature of this ’dark matter‘ is unknown, except that it cannot be made of normal stars, dust or gas, as they would be easily detected. Exotic particles such as axions, massive neutrinos or other weakly interacting massive particles (collectively known as WIMPs) have been proposed3,4, but have yet to be detected. A less exotic alternative is normal matter in the form of bodies with masses ranging from that of a large planet to a few solar masses. Such objects, known collectively as massive compact halo objects5 (MACHOs), might be brown dwarfs or ‘jupiters’ (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. We are conducting a microlensing experiment to determine whether the dark matter halo of our Galaxy is made up of MACHOs. Here we report a candidate for such a microlensing event, detected by monitoring the light curves of 1.8 million stars in the Large Magellanic Cloud for one year. The light curve shows no variation for most of the year of data taking, and an upward excursion lasting over 1 month, with a maximum increase of ~2 mag. The most probable lens mass, inferred from the duration of the candidate lensing event, is ~0.1 solar mass.

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