Possible gravitational microlensing of a star in the Large Magellanic Cloud

THERE is now abundant evidence for the presence of large quantities of unseen

matter surrounding normal galaxies, including our own^1,2. 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 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 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.