Gravitational microlensing

Gravitational microlensing is an astronomical technique used to detect planet - stellar mass objects in space using the gravitational lens effect. Typically, astronomers can only detect bright objects that emit lots of light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a tiny fraction of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.

When a nearby object passes in front of a distant star or quasar, the gravity of the nearby object magnifies the distant star. By detecting that magnification, astronomers can study the near star.

Microlensing allows astronomers to study massive objects no matter how faint. It is thus an ideal technique to study the galactic population of such faint or dark objects as brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes, and Massive Compact Halo Objects.

Microlensing has been used to search for dark matter in the Milky Way and other galaxies, to hunt for planets around stars at the center of the Milky Way and to study limb darkening on distant stars.

How it works

Microlensing is based on the gravitational lens effect. A massive object (the lens) will bend the light of a bright background object (the source). This can generate multiple distorted, magnified, and brightened images of the background source. Microlensing can be distinguished from other gravitational lenses ("macrolenses") because it deals with small lens masses requiring a very different observational approach to detect.

The name "Microlensing" stands in apposition to another type of gravitational lensing ("macrolensing") where the same physical effect is studied using different observational techniques. When the lens is very massive (a "macrolens"), a galaxy or cluster of galaxies, the bending of light by the lens can be large enough (~ 1 arcsecond) to see with a high resolution telescope such as the Hubble Space Telescope. When the lens has low mass (a microlens), such as a single star, the light will bend by only ~ 1 millionth of a degree -- too small to be detected except with heroic measures. However, a low mass lens will pass in front of the source in a reasonable amount of time, seconds to years instead of millions of years. The source will change its apparent brightness and other observable features over time, and these changes can be monitored to detect and study the event. A microlens is thus a gravitational lens in which the lens can be practically observed to change in time.

For these typical lenses, only one physical parameter can be extracted: the lens time scale which is related to the lens mass, distance, and velocity. Since so little information can be measured from photometry, heroic measures are currently underway to measure additional effects of the gravitational lens including measuring a tiny shift in apparent position of the source (astrometric microlensing) and even resolving the separate images of the event (interferometric microlensing) [http://adsabs.harvard.edu/cgi-bin/bib_query?2001A%26A...375..701D.

In practice, microlensing is very rare. The lens will magnify the source only when the two stars are nearly perfectly aligned. Even in the densest fields of stars, such as the galactic center, only about one in a million stars will microlensed at any particular time. This fraction is known as the microlensing optical depth. To have any chance of detecting a microlensing event, a microlensing experiment will monitor the apparent magnitude of millions of source stars on a regular basis. Some 2% of these stars are naturally variable stars which change their absolute magnitude and have to be weeded out to find true lensing events. A tiny fraction of stars will appear to brighten because some dark lens is passing in front of them. By observing this brightening, astronomers can infer the existence of the dark lens. The experiment which detects the lens often alerts its discovery, and other specialised experiments then follow the lens more intensively hoping to find small deviations from the typical light curve.

History

The bending of a light ray by a massive object was first worked out by Einstein as part of the general theory of relativity. The first measurement of a gravitational microlens supervised by Arthur Eddington during a solar eclipse. He measured the small shift in the apparent position of stars due the gravity of the sun, dramatically confirming the predictions of general relativity. In 1936 in the journal Science, Einstein worked out the magnification of a source by a lens star, but concluded that "there is no great chance of observing this phenomenon".

In 1969, lensing of a distant quasar by stars in the Milky Way was proposed by Byalko, but this paper was not followed up. In 1979 and 1984, Chang and Refsdal discussed microlensing of a distant quasar by stars in another galaxy along the line of sight. They worked out much of the mathematics of non-standard microlensing.

In 1986, Polish astronomer Bohdan Paczyński of Princeton University first proposed using microlensing to look for dark matter, the unseen material that is thought to dominate the universe. Two groups of particle physicists worried about dark matter heard his talks and worked with astronomers to formed the Anglo-Australian MACHO and the French EROS ^* which searched for events in the Galactic bulge using photographic plates at the 1.3 m Swope telescope in Las Campanas Observatory, Chile.

The first microlensing events towards the Large Magellanic Cloud which might be indicative of dark matter were reported in back to back Nature papers by the MACHO and EROS solar masses ^*." target="_blank" > If real, it would suggest a major change in our view of the universe since there is no good candidate object with the right mass to explain this measurement ^*. The cause of the MACHO measurement--whether a detection of dark matter, ordinary stars, supernovae, or a statistical fluke-- is still uncertain.

Mathematics

Exotic Microlensing

Detection of Extrasolar Planets

If the lensing object is a star with a planet orbiting it (for example), then the planet can be detected as an additional microlensing event on top of that caused by the star. From this, information about the planet - such as its mass and distance from the star - can be detected.

This method of detecting extrasolar planets has the advantage over the transit method as the detection of events has a reduced dependency on the size of the planet and the distance between the planet and its host star.

Future Development of Microlensing

Microlensing experiments

There are two basic types of microlensing experiments. "Search" groups use large-field images to find new microlensing events. "Follow-up" groups often coordinate telescopes around the world to provide intensive coverage of select events. The initial experiments all had somewhat risqué names until the formation of the PLANET group led by Penny Sackett. There are current proposals to build new specialized microlensing satellites, or to use other satellites to study microlensing.

Search Collaborations

Dark Unseen Objects (DUO) (? - 1996?) Photographic plate search of bulge. Remarkable for largely being the work of a single graduate student, Christophe Alard, for his Ph.D. Thesis.
Experience de Recherche des Objets Sombres (EROS) (1993-2002) Largely French collaboration. EROS1: Photographic plate search of LMC: EROS2: CCD search of LMC, SMC, Bulge & spiral arms.
MACHO (1993 - 1999) Australia & US collaboration. CCD search of bulge and LMC.
Optical Gravitational Lensing Experiment (OGLE) Polish collaboration.
Microlensing Observations in Astrophysics (MOA) Japanese-New Zealand collaboration
SuperMACHO (2001 - ), successor to the MACHO collaboration used 4 m CTIO telescope to study faint LMC microlenses.