POSSIBLE GRAVITATIONAL MICROLENSING OF A STAR IN THE LARGE MAGELLANICCLOUD

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. Exoti c particles such as axions, massive neutrinos or other weakly interact ing massive particles (collectively known as WIMPs) have been proposed 3,4, 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 mass ive compact halo objects5 (MACHOs), might be brown dwarfs or 'jupiters ' (bodies too small to produce their own energy by fusion), neutron st ars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the ap parent brightness of background stars in nearby galaxies. We are condu cting a microlensing experiment to determine whether the dark matter h alo 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 lig ht curve shows no variation for most of the year of data taking, and a n upward excursion lasting over 1 month, with a maximum increase of ap proximately 2 mag. The most probable lens mass, inferred from the dura tion of the candidate lensing event, is approximately 0.1 solar mass.