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Micro Lensing

Page history last edited by Adam Amara 15 years, 3 months ago

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 In Microlensing the lenses are compact objects with masses ranging from the stellar to the planetary regime. The images of background sources created by such kind of lenses are in general far to small to be observed directly. In a typical scenario within our Milky Way, the images of a background star in the Galactic Bulge (i.e. close to the center of our galaxy) created by anothers star acting as lens passing halfway in the line of sight are separated by less than a milliarcsecond, which is still beyond the resolving power of current optical telescopes. However due to the relative motion of observer, lens and source the magnification depending on the projected alignment between lens and source changes in time thus causing an apparent transient brightening of the source, a socalled microlensing event light curve, which can easily be detected by even rather small ( 0.5 -1 m class) telescopes.


Following an idea proposed by Paczynski in 1986 to use microlensing as a way to search for compact dark matter objects in the halo of our galaxy and thanks to advances in ccd detectors teams such as MACHO (MAssive Compact Halo Objects, Alcock et al. 1993) and EROS (Experience pour la Recherche d'Objects Sombres, Aubourg et al. 1993) formed to detect microlensing events in the direction towards of the Magellanic clouds, which stars were espected to get microlensed by dark lenses (if they exist) residing in the halo.


However the lack of detected events (which in addition most likely were caused by normal stars rather than dark matter objects) indicates that at in our halo the dark matter is not present in a compact form, at least not as dark brown warfs or more massive compact bodies. The cross section for microlensing to occur is small (on the order of one to a million) and to be sure that their experimental setup is working microlensing astronomers in the mid 1990s redirected their search towards the Galactic Bulge. Here the density of of both background stars and ordinary stars in the line of sight is so high that microlensing is guaranteed to occur. Indeed this time microlensing events were discovered at a much higher rate than towards the Magellanic clouds. The time scales of these Bulge microlensing events are on the order of weeks to months.


Motivated by these discoveries and by another brillant idea by Paczynski who together with Mao showed that planets orbiting the lens can cause detectable features in microlensing light curves, survey teams such as OGLE (ogle.astrouw.edu.pl) and MOA (www.phys.canterbury.ac.nz/moa/) formed to monitor the brightness of several million stars in the Bulge (first on a weekly to daily basis, now down to hourly or even higher sampling) and to issue public alerts for ongoing microlensing events so that other teams such as PLANET (www.planet.iap.fr) and Microfun (www.astronomy.ohio-state.edu/~microfun) with dedicated networks of telescopes can follow up selected events on a continous round-the-clock base to look for the very short lived planetary light curve signatures (lasting days for Jupiters and just a few hours for Earth Mass planets).




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