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How often does microlensing occur in the Milky Way?

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The Milky Way and our nearest galactic neighbors, The Large and Small Magellanic Clouds (LMC and SMC), are filled with billions of stars and possibly other compact objects that can act as gravitational lenses when they are situated between us and some background source. These same objects can also act as sources, if they are luminous and lie behind a lens. Despite this plethora of lenses and sources, nearly 60 years separated the proposition by Einstein in 1936 (Einstein 1936) that Galactic microlensing may take place (though he thought it would be impossible to detect), and its firm discovery in 1993. The reason for the delay between concept and discovery is that a very precise alignment between lens and source is required for a detectable change in brightness. In order to produce a change in apparent brightness of a few percent, the angular separation must be comparable to the size of a typical Einstein ring radius --- or about 1 milliarcsecond of arc. Even in the regions of the Galactic Center, where the stellar densities are the highest, the chance that at any given time a star is aligned that precisely is only about 1 in one million. This means that at least one million stars must be carefully monitored in order to have a reasonable chance of detecting the brightening and dimming that is characteristic to microlensing. Since the line-of-sight to the Magellanic Clouds intersects fewer Milky Way stars, the chances are even smaller that microlensing would be observed for LMC or SMC sources, requiring that even more stars be monitored.

The possibility to take and analyze data from one million stars has been made possible only very recently with the advent of large modern electronic detectors coupled to fast analysis computers. Even so, the task is daunting. What could convince teams of astronomers to spend several years monitoring millions of stars every night in search of a few microlensing events in a haystack of normal stellar light curves? In 1986, Bohdan Paczynski, an astronomer at Princeton University in the United States, speculated that if the dark matter that is believed to be present in ample quantity in the Milky Way was comprised of compact objects capable of lensing background luminous objects, then the chance of observing microlensing might be larger by about a factor of 10 than previously thought (Paczynski 1986). The composition of the dark matter, believed to compose 50-95% of the total mass of the Milky Way, is unknown, but its presence is inferred from the large velocities at the outskirts of the Galaxy that can only be caused by a formidable gravitational tug. The nature of dark matter, its distribution and composition, is one of the outstanding mysteries of modern astronomy; perhaps microlensing would provide a way to solve part of the mystery.

With the primary motivation of looking for dark matter, three teams of astronomers began to search for microlensing by stars and (possible) dark lenses in the Milky Way. A Polish-American collaboration, OGLE, (Optical Gravitational Lensing Experiment) began monitoring stars in the direction of the Galactic Center. A French team (EROS, Experience pour la Recherche d'Objets sombres) and an American-Australian team (MACHO, MAssive Compact Halo Objects collaboration) concentrated their efforts along the lines-of-sight toward the Magellanic Clouds. After a few years, each of these teams announced independently the discovery of a different microlensing event (Udalski et al. 1993, Aubourg et al, 1993, Alcock et al. 1993). The light curves of one of the background sources that each team was monitoring showed the characteristic microlensing shape and was ``achromatic,'' as expected (Fig. 5).

Fig. 5 --- Above: The first microlensing event reported by the MACHO collaboration toward the LMC. The source star did not vary for more than a year and then suddenly increased in apparent brightness by a factor of 7 before returning to its baseline brightness, which it has maintained ever since. Below: The first microlensing event reported by the EROS collaboration toward the SMC. The MACHO team also discovered this same event independently. Both the LMC and SMC events are achromatic: the light curves are the same in blue and red filters. Time scales are in days. Click on the figures for a zoom. (Courtesy MACHO and EROS.)

Since these first discoveries, a few tens of microlensing events have been discovered in the direction of the LMC, and in 1997 the first event in the direction of the SMC was reported by both the MACHO and EROS teams (Fig. 5). Toward the center of our own Galaxy, hundreds of events have now been detected; in fact, at any given moment, at least 5 to 10 microlensing events are known to on-going somewhere in the bulge of the Milky Way (Fig. 6).

Fig. 6 --- At any given moment, several known microlensing events are on-going in the direction of the bulge of our own Galaxy. These events, shown as they occured in time after April 1, 1995 were detected by the MACHO team in real-time and reported to the community electronically. Click on figure for a zoom.

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