Possible gravitational microlensing of a star in the Large Magellanic Cloud

Abstract

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. In 1986, Bohdan Paczynski suggested that dark, compact objects in the halo of our Galaxy — the local cousins of similar objects conjectured to contribute dark matter to the haloes of other galaxies — could be detected on Earth, as their gravitation might occasionally amplify the light arriving from more distant stars. In 1993, in surveys of several million stars in the Large Magellanic Cloud, C. Alcock et al. and E. Aubourg et al. observed several candidate events. In total, the two groups detected three stars for which the brightness increased by two magnitudes over an interval of roughly one month.