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Gravitational microlensing is an Astronomical technique taking advantage of the Gravitational Lens ing effect of a foreground compact mass, such as a Planet or Star , as it passes very close to the line of sight to a more distant source, such as a star or a Quasar . 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. It can also be used 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 Object s. INTRODUCTION The distinguishing characteristic between microlensing and other cases of gravitational lensing is that the multiple distorted images of the source which the lens may induce are not individually Resolvable by optical Telescopes . This is because the scale of the images is typically of the order of Arcsecond s for a gravitational lens (which can be detected fairly easily by the Very Large Array at radio frequencies, and the Hubble Space Telescope at optical frequencies), while microlensing is typically on the order of Microarcsecond s. Instead, with microlensing we observe a single apparent image with a brightness given by the combined brightness of the individual images. Microlensing is detectable when the lens traverses close by the source, producing a transient Magnification in the source brightness, typically lasting between 20 and 200 days. The magnification of the source increases as the separation on the sky between the lens and source decreases. The Probability of observing a magnification of 50% or more due to stars in our Milky Way galaxy microlensing each other is roughly one in a million towards the Milky Way Bulge . Source brightness is the only data we can acquire during gravitational microlensing event. Hence we produce a Light Curve - a brightness as a function of time. Typical event light curve looks as follows: Because microlensing depends only upon the mass of the lens, and not on its brightness, it can be used for a range of applications in astronomy. Examples include surveys for Dark Matter in the form of MACHOs , surveys of the structure and composition of the inner regions of the Milky Way, or the detection of Extrasolar Planets . Over more than a decade of monitoring millions of stars towards the Milky Way bulge, a number of surveys have collectively detected thousands of microlensing events, including some involving Binary lens systems, of which a few are planetary systems. Detailed Photometric measurements of the Light Curve have also been used to resolve the Photosphere of a few source stars. HISTORY Gravitational microlensing has a checkered past. In 1986 , Polish astronomer Bohdan Paczyński of Princeton University first proposed using it to look for mysterious Dark Matter , the unseen material that is thought to dominate the universe. In 1991 he suggested it might be used to find planets. Successes with the gravity lensing method date back to 2002 , when a group of Polish astronomers ( Andrzej Udalski , Marcin Kubiak and Michał Szymański from Warsaw , and Bohdan Paczyński) during project OGLE (the Optical Gravitational Lensing Experiment ) perfected a workable method. During one month they claimed to find objects, many of which could be planets. Since then, four extrasolar planets have been detected using microlensing, and this technique is viewed as one of the most promising methods for finding Earth-mass planets around sun-like stars. MATHEMATICS EXOTIC MICROLENSING Detection of Extrasolar Planets If the lensing object is a star with a planet orbitting 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
Follow-up Collaborations
Andromeda Galaxy Pixel Lensing Collaborations
Proposed satellite experiments
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