Gravitational Microlensing

What is microlensing?

Gravitational microlensing refers to the apparent brightening of a background source caused by a lensing object located sufficiently close to the line of sight. This gravitational focusing effect does not require the intervening object to be luminous, and hence it has been suggested as a way to detect astrophysical dark matter candidates (such as black holes, an example is shown in the above figure: image credit NASA). In the Galaxy, microlensing can be used to detect the so-called MAssive Compact Halo Objects (MACHOs) in the Galactic halo ranging from 1/10,000,000 solar masses to one million solar masses. A good introduction can be found at the PLANET collaboration web site.

Discovery of extrasolar planets with microlensing

The most exciting results from various microlensing surveys (see below) are obviously the discovery of extrasolar planets, a method proposed by myself and Bohdan Paczynski in 1991. So far about 50 microlensing events have been discovered. With surveys like OGLE MOA and KMTNet, more and more planets will be discovered in the next decade, by which time hopefully WFIRST will have started to discover extrasolar planets at a rate that is one order of magnitude higher than ground-based experiments. For detailed discussions on how this method works, see Gaudi's review.

A 5.5 Earth planet discovered by the PLANET collaboration (Beaulieu et al. 2006).

Microlensing survey and followup teams

Microlensing Searches toward the Galactic Bulge, Magellanic Clouds and Spiral Arms:

OGLE (Optical Gravitational Lensing Experiment)
MOA (Microlensing Observations in Astrophysics)
KMTNet a newcomer in microlensing that will be increasingly important (perhaps dominant). This Korean survey installed three 1.6m telescopes in Australia, Chile and South Africa. It will sample four areas of sky, each with 4 square degrees of field of view every 15 minutes. It has the potential to combine survey and followup efforts.
MACHO (MAssive Compact Halo Object), an influential survey at the beginning of 2000's.
EROS (Experience pour la Recherche d'Objets sombres), another influential survey at the beginning of 2000's.

Most events are now discovered in real-time, see OGLE IV Early-Warning System and MOA Alerts. These in turn allow followup teams to study interesting events in much greater detail. Some of the teams are listed here:

microFun, a survey led by Andy Gould and his collaborators
PLANET (Probing Lensing Anomalies NETwork)
ARTEMIS, a followup effort led by Martin Dominik (I am apparently a legacy member)
RoboNet (of which I am a current member)
MPS (Microlensing Planet Search)

There are also microlensing surveys towards M31, but these turn out to be disappointing, discoverying very few events in the last two decades.

Reviews on Microlensing

To learn more about microlensing, you can read the following reviews.

Paczynski, who started the field, wrote an excellent review on microlensing.
My own 1999 and 2012 reviews.
see Gaudi's review on extrasolar planet microlensing
Roulet, E. and Mollerach, S. (1997), Phys. Reports, 279, 67
Gould (1996) contains a very good review on the mass functions
There are also two nice reviews on microlensing in the book Unsolved Problems In Astrophysics, eds. John Bahcall and Jerry Ostriker, by Gould (1997, p. 241), and by Alcock (1997, p. 253). The latter discusses why microlensing surveys are different from the traditional way of doing astrophysical research
For a view on microlensing from Australia, see York's article

This page is under construction. Last updated on Sept. 17, 2016