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75-408: With the flexible scheduling and short response time of robotic telescopes being ideal for time-domain astronomy , RoboNet-1.0 had two major science goals that critically depend on these requirements: the determination of origin and nature of gamma-ray bursts , and the detection of cool extra-solar planets by means of gravitational microlensing . Apart from their science use, the telescopes forming

150-444: A brown dwarf ), about 13.4 times the mass of Jupiter , was reported. Comparing this method of detecting extrasolar planets with other techniques such as the transit method, one advantage is that the intensity of the planetary deviation does not depend on the planet mass as strongly as effects in other techniques do. This makes microlensing well suited to finding low-mass planets. It also allows detection of planets further away from

225-421: A clear buffer between the radiation from the lens and source objects. It magnifies the distant source, revealing it or enhancing its size and/or brightness. It enables the study of the population of faint or dark objects such as brown dwarfs , red dwarfs , planets , white dwarfs , neutron stars , black holes , and massive compact halo objects . Such lensing works at all wavelengths, magnifying and producing

300-443: A deviation from the traditional microlensing curve that lasts as long as the time for the lens to cross the source, known as a finite source light curve . The length of this deviation can be used to determine the time needed for the lens to cross the disk of the source star t S {\displaystyle t_{S}} . If the angular size of the source θ S {\displaystyle \theta _{S}}

375-466: A massive compact foreground object, the bending of light due to its gravitational field, as discussed by Albert Einstein in 1915, leads to two distorted images (generally unresolved ), resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object. Ideally aligned microlensing produces

450-488: A means to find dark objects like brown dwarfs and black holes, study starspots , measure stellar rotation, and probe quasars including their accretion disks . Microlensing was used in 2018 to detect Icarus , then the most distant star ever observed. 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

525-402: A microlensing event in progress has been identified, the monitoring program that detects it often alerts the community to its discovery, so that other specialized programs may follow the event more intensively, hoping to find interesting deviations from the typical light curve. This is because these deviations – particularly ones due to exoplanets – require hourly monitoring to be identified, which

600-478: A microlensing event, the brightness of the source is amplified by an amplification factor A. This factor depends only on the closeness of the alignment between observer, lens, and source. The unitless number u is defined as the angular separation of the lens and the source, divided by θ E {\displaystyle \theta _{E}} . The amplification factor is given in terms of this value: This function has several important properties. A(u)

675-472: A short period of time. The relevant time scale is called the Einstein time t E {\displaystyle t_{E}} , and it's given by the time it takes the lens to traverse an angular distance θ E {\displaystyle \theta _{E}} relative to the source in the sky. For typical microlensing events, t E {\displaystyle t_{E}}

750-468: A well known high energy electromagnetic transient. The proposed ULTRASAT satellite will observe a field of more than 200 square degrees continuously in an ultraviolet wavelength that is particularly important for detecting supernovae within minutes of their occurrence. Gravitational microlensing Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from

825-509: A wide range of possible warping for distant source objects that emit any kind of electromagnetic radiation. Microlensing by an isolated object was first detected in 1989. Since then, microlensing has been used to constrain the nature of the dark matter , detect exoplanets , study limb darkening in distant stars, constrain the binary star population, and constrain the structure of the Milky Way's disk. Microlensing has also been proposed as

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900-411: Is a range of alignments where two additional images are created. These alignments are known as caustics . At these alignments, the magnification of the source is formally infinite under the point-source approximation. Caustic crossings in binary lenses can happen with a wider range of lens geometries than in a single lens. Like a single lens source caustic, it takes a finite time for the source to cross

975-462: Is always greater than 1, so microlensing can only increase the brightness of the source star, not decrease it. A(u) always decreases as u increases, so the closer the alignment, the brighter the source becomes. As u approaches infinity, A(u) approaches 1, so that at wide separations, microlensing has no effect. Finally, as u approaches 0, for a point source A(u) approaches infinity as the images approach an Einstein ring. For perfect alignment (u = 0), A(u)

1050-429: Is characterized by a value known as the optical depth due to microlensing. (This is not to be confused with the more common meaning of optical depth , although it shares some properties.) The optical depth is, roughly speaking, the average fraction of source stars undergoing microlensing at a given time, or equivalently the probability that a given source star is undergoing lensing at a given time. The MACHO project found

1125-480: Is currently uncertain as to whether there is any halo microlensing excess that could be due to dark matter at all. The SuperMACHO project currently underway seeks to locate the lenses responsible for MACHO's results. Despite not solving the dark matter problem, microlensing has been shown to be a useful tool for many applications. Hundreds of microlensing events are detected per year toward the Galactic bulge , where

1200-461: Is in contrast to the timescale of the millions or billions of years during which the galaxies and their component stars in our universe have evolved. Singularly, the term is used for violent deep-sky events, such as supernovae , novae , dwarf nova outbursts, gamma-ray bursts , and tidal disruption events , as well as gravitational microlensing . Time-domain astronomy also involves long-term studies of variable stars and their changes on

1275-401: Is known as a light curve . A typical microlensing light curve is shown below: A typical microlensing event like this one has a very simple shape, and only one physical parameter can be extracted: the time scale, which is related to the lens mass, distance, and velocity. There are several effects, however, that contribute to the shape of more atypical lensing events: Most focus is currently on

1350-449: Is known, the Einstein angle can be determined as These measurements are rare, since they require an extreme alignment between source and lens. They are more likely when θ S / θ E {\displaystyle \theta _{S}/\theta _{E}} is (relatively) large, i.e., for nearby giant sources with slow-moving low-mass lenses close to the source. In finite source events, different parts of

1425-545: Is longer than that of the magnification, and can be used to find the mass of the lens. In 2022 it was reported that this technique was used to make the first unambiguous detection of an isolated stellar-mass black hole , using observations by the Hubble Space Telescope stretching over six years, starting in August 2011 shortly after the microlensing event was detected. The black hole has a mass of about 7 times

1500-503: Is on the order of a few days to a few months. The function u(t) is simply determined by the Pythagorean theorem: The minimum value of u, called u min , determines the peak brightness of the event. In a typical microlensing event, the light curve is well fit by assuming that the source is a point, the lens is a single point mass, and the lens is moving in a straight line: the point source-point lens approximation. In these events,

1575-405: Is so small, it is not generally observed for a typical microlensing event, but it can be observed in some extreme events as described below. Although there is no clear beginning or end of a microlensing event, by convention the event is said to last while the angular separation between the source and lens is less than θ E {\displaystyle \theta _{E}} . Thus

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1650-472: Is the Einstein timescale t E {\displaystyle t_{E}} . However, in some cases, events can be analyzed to yield the additional parameters of the Einstein angle and parallax: θ E {\displaystyle \theta _{E}} and π E {\displaystyle \pi _{E}} . These include very high magnification events, binary lenses, parallax, and xallarap events, and events where

1725-404: Is theoretically infinite. In practice, real-world objects are not point sources, and finite source size effects will set a limit to how large an amplification can occur for very close alignment, but some microlensing events can cause a brightening by a factor of hundreds. Unlike gravitational macrolensing where the lens is a galaxy or cluster of galaxies, in microlensing u changes significantly in

1800-937: The Gravitational-wave Optical Transient Observer (GOTO) began looking for collisions between neutron stars. The ability of modern instruments to observe in wavelengths invisible to the human eye ( radio waves , infrared , ultraviolet , X-ray ) increases the amount of information that may be obtained when a transient is studied. In radio astronomy the LOFAR is looking for radio transients. Radio time domain studies have long included pulsars and scintillation. Projects to look for transients in X-ray and gamma rays include Cherenkov Telescope Array , eROSITA , AGILE , Fermi , HAWC , INTEGRAL , MAXI , Swift Gamma-Ray Burst Mission and Space Variable Objects Monitor . Gamma ray bursts are

1875-468: The Hubble Space Telescope . With microlensing, the lens mass is too low (mass of a planet or a star) for the displacement of light to be observed easily, but the apparent brightening of the source may still be detected. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. As the alignment changes, the source's apparent brightness changes, and this can be monitored to detect and study

1950-644: The MACHO Project . These efforts, beside the discovery of the microlensing events itself, resulted in the orders of magnitude more variable stars known to mankind. Subsequent, dedicated sky surveys such as the Palomar Transient Factory , the spacecraft Gaia and the LSST , focused on expanding the coverage of the sky monitoring to fainter objects, more optical filters and better positional and proper motions measurement capabilities. In 2022,

2025-657: The Nancy Grace Roman Space Telescope and the Vera C. Rubin Observatory . The mathematics of microlensing, along with modern notation, are described by Gould and we use his notation in this section, though other authors have used other notation. The Einstein radius , also called the Einstein angle, is the angular radius of the Einstein ring in the event of perfect alignment. It depends on

2100-593: The Optical Gravitational Lensing Experiment , which began searching for events in the direction of the Galactic bulge . The first two microlensing events in the direction of the Large Magellanic Cloud that might be caused by dark matter were reported in back to back Nature papers by MACHO and EROS in 1993, and in the following years, events continued to be detected. During this time, Sun Hong Rhie worked on

2175-468: The solar mass and is about 1.6 kiloparsecs (5.2 kly) away, in Sagittarius , while the star is about 6 kiloparsecs (20 kly) away. There are millions of isolated black holes in our galaxy, and being isolated very little radiation is emitted from their surroundings, so they can only be detected by microlensing. The authors expect that many more will be found with future instruments, specifically

2250-574: The GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) . When the two images of the source are not resolved (that is, are not separately detectable by the available instruments), the measured position is an average of the two positions, weighted by their brightness. This is called the position of the centroid . If the source is, say, far to the "right" of the lens, then one image will be very close to

2325-842: The RoboNet-1.0 have also been made available for two educational programmes, the Faulkes Telescope Project and the National Schools‘ Observatory . The RoboNet microlensing programme, led by the University of St Andrews , engages in a common campaign with the PLANET collaboration since 2005. With the official end of RoboNet-1.0 in October 2007, and the earlier acquisition of the two Faulkes Telescopes by Las Cumbres Observatory Global Telescope Network ,

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2400-472: The aim of detecting extrasolar planets. These include MiNDSTEp, RoboNet, MicroFUN and PLANET. In September 2020, astronomers using microlensing techniques reported the detection , for the first time, of an earth-mass rogue planet unbounded by any star, and free floating in the Milky Way galaxy . Microlensing not only magnifies the source but also moves its apparent position. The duration of this

2475-543: The alignment needed is so precise and difficult to predict, microlensing is very rare. Events, therefore, are generally found with surveys , which photometrically monitor tens of millions of potential source stars, every few days for several years. Dense background fields suitable for such surveys are nearby galaxies, such as the Magellanic Clouds and the Andromeda galaxy, and the Milky Way bulge. In each case,

2550-446: The amount of deflection of a light ray from a star under Newtonian gravity. In 1915 Albert Einstein correctly predicted the amount of deflection under General Relativity , which was twice the amount predicted by von Soldner. Einstein's prediction was validated by a 1919 expedition led by Arthur Eddington , which was a great early success for General Relativity. In 1924 Orest Chwolson found that lensing could produce multiple images of

2625-407: The background source. Microlensing is caused by the same physical effect as strong gravitational lensing and weak gravitational lensing but it is studied by very different observational techniques. In strong and weak lensing, the mass of the lens is large enough (mass of a galaxy or galaxy cluster) that the displacement of light by the lens can be resolved with a high resolution telescope such as

2700-425: The caustic. If this caustic-crossing time t S {\displaystyle t_{S}} can be measured, and if the angular radius of the source is known, then again the Einstein angle can be determined. As in the single lens case when the source magnification is formally infinite, caustic crossing binary lenses will magnify different portions of the source star at different times. They can thus probe

2775-408: The chances of looking in the right place at the right time were low. Schmidt cameras and other astrographs with wide field were invented in the 20th century, but mostly used to survey the unchanging heavens. Historically time domain astronomy has come to include appearance of comets and variable brightness of Cepheid-type variable stars . Old astronomical plates exposed from the 1880s through

2850-620: The early 1990s held by the Harvard College Observatory are being digitized by the DASCH project. The interest in transients has intensified when large CCD detectors started to be available to the astronomical community. As telescopes with larger fields of view and larger detectors come into use in the 1990s, first massive and regular survey observations were initiated - pioneered by the gravitational microlensing surveys such as Optical Gravitational Lensing Experiment and

2925-423: The event duration is determined by the time it takes the apparent motion of the lens in the sky to cover an angular distance θ E {\displaystyle \theta _{E}} . The Einstein radius is also the same order of magnitude as the angular separation between the two lensed images, and the astrometric shift of the image positions throughout the course of the microlensing event. During

3000-440: The event. Thus, unlike with strong and weak gravitational lenses, microlensing is a transient astronomical event from a human timescale perspective, thus a subject of time-domain astronomy . Unlike with strong and weak lensing, no single observation can establish that microlensing is occurring. Instead, the rise and fall of the source brightness must be monitored over time using photometry . This function of brightness versus time

3075-432: The existence and determine the mass and separation of the planet around the lens. Deviations typically last a few hours or a few days. Because the signal is strongest when the event itself is strongest, high-magnification events are the most promising candidates for detailed study. Typically, a survey team notifies the community when they discover a high-magnification event in progress. Follow-up groups then intensively monitor

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3150-869: The form of massive compact halo objects ( MACHOs ) in the Galactic halo , by observing background stars in a nearby galaxy. Two groups of particle physicists working on dark matter heard his talks and joined with astronomers to form the Anglo-Australian MACHO collaboration and the French EROS collaboration. In 1986, Robert J. Nemiroff predicted the likelihood of microlensing and calculated basic microlensing induced light curves for several possible lens-source configurations in his 1987 thesis. In 1991 Mao and Paczyński suggested that microlensing might be used to find binary companions to stars, and in 1992 Gould and Loeb demonstrated that microlensing can be used to detect exoplanets. In 1992, Paczyński founded

3225-583: The forthcoming LSST at the Vera C. Rubin Observatory . Time-domain astronomy studies transient astronomical events (" transients "), which include various types of variable stars, including periodic , quasi-periodic , high proper motion stars, and lifecycle events ( supernovae , kilonovae ) or other changes in behavior or type. Non-stellar transients include asteroids , planetary transits and comets . Transients characterize astronomical objects or phenomena whose duration of presentation may be from milliseconds to days, weeks, or even several years. This

3300-490: The growth of a new field of astrophysics research, time-domain astronomy , which studies the variability of brightness and other parameters of objects in the universe in different time scales." Also the 2017 Dan David Prize was awarded to the three leading researchers in the field of time-domain astronomy: Neil Gehrels ( Swift Gamma-Ray Burst Mission ), Shrinivas Kulkarni ( Palomar Transient Factory ), Andrzej Udalski ( Optical Gravitational Lensing Experiment ). Before

3375-653: The invention of telescopes , transient events that were visible to the naked eye , from within or near the Milky Way Galaxy, were very rare, and sometimes hundreds of years apart. However, such events were recorded in antiquity, such as the supernova in 1054 observed by Chinese, Japanese and Arab astronomers, and the event in 1572 known as " Tycho's Supernova " after Tycho Brahe , who studied it until it faded after two years. Even though telescopes made it possible to see more distant events, their small fields of view – typically less than 1 square degree – meant that

3450-435: The lens and source by It is mathematically convenient to use the inverses of some of these quantities. These are the Einstein proper motion and the Einstein parallax These vector quantities point in the direction of the relative motion of the lens with respect to the source. Some extreme microlensing events can only constrain one component of these vector quantities. Should these additional parameters be fully measured,

3525-421: The lens is visible. Although the Einstein angle is too small to be directly visible from a ground-based telescope, several techniques have been proposed to observe it. If the lens passes directly in front of the source star, then the finite size of the source star becomes an important parameter. The source star must be treated as a disk on the sky, not a point, breaking the point-source approximation, and causing

3600-498: The lens mass M, the distance of the lens d L , and the distance of the source d S : For M equal to 60 Jupiter masses , d L = 4000 parsecs, and d S = 8000 parsecs (typical for a Bulge microlensing event), the Einstein radius is 0.00024 arcseconds ( angle subtended by 1 au at 4000 parsecs). By comparison, ideal Earth-based observations have angular resolution around 0.4 arcseconds, 1660 times greater. Since θ E {\displaystyle \theta _{E}}

3675-412: The lens population studied comprises the objects between Earth and the source field: for the bulge, the lens population is the Milky Way disk stars, and for external galaxies, the lens population is the Milky Way halo, as well as objects in the other galaxy itself. The density, mass, and location of the objects in these lens populations determines the frequency of microlensing along that line of sight, which

3750-573: The lens star, the mass of the planet and its orbital distance can be estimated. The first success of this technique was made in 2003 by both OGLE and MOA of the microlensing event OGLE 2003–BLG–235 (or MOA 2003–BLG–53) . Combining their data, they found the most likely planet mass to be 1.5 times the mass of Jupiter. As of April 2020, 89 exoplanets have been detected by this method. Notable examples include OGLE-2005-BLG-071Lb , OGLE-2005-BLG-390Lb , OGLE-2005-BLG-169Lb , two exoplanets around OGLE-2006-BLG-109L , and MOA-2007-BLG-192Lb . Notably, at

3825-439: The mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light ( stars ) or large objects that block background light (clouds of gas and dust). These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light. When a distant star or quasar gets sufficiently aligned with

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3900-678: The microlensing optical depth (due to stars in the Galactic disk) is about 20 times greater than through the Galactic halo. In 2007, the OGLE project identified 611 event candidates, and the MOA project (a Japan-New Zealand collaboration) identified 488 (although not all candidates turn out to be microlensing events, and there is a significant overlap between the two projects). In addition to these surveys, follow-up projects are underway to study in detail potentially interesting events in progress, primarily with

3975-542: The microlensing programme is carried on as RoboNet-II. Starting in 2008, RoboNet-II has been using the expert system for microlensing anomaly detection that is being provided by the Automated Robotic Terrestrial Exoplanet Microlensing Search (ARTEMiS). RoboNet-II aims at obtaining a first census of cool terrestrial exoplanets . RoboNet data have contributed to the detection of several extra-solar planets (in

4050-408: The more unusual microlensing events, especially those that might lead to the discovery of extrasolar planets. Another way to get more information from microlensing events involves measuring the astrometric shifts in the source position during the course of the event and even resolving the separate images with interferometry . The first successful resolution of microlensing images was achieved with

4125-523: The motion of the lens while the difference in the time of peak amplification yields the component parallel to the motion of the lens. This direct measurement has been reported using the Spitzer Space Telescope . In extreme cases, the differences may even be measurable from small differences seen from telescopes at different locations on Earth, i.e. terrestrial parallax. The Einstein parallax can also be measured through orbital parallax;

4200-675: The motion of the observer, caused by the rotation of the Earth about the Sun and the Sun through the Galaxy means that a microlensing event is being observed from different angles at each observation epoch. This was first reported in 1995 and has been reported in a handful of events since. Parallax, in point-lens events, can best be measured for long-timescale events, with a large π E {\displaystyle \pi _{E}} , i..e. from slow-moving, low mass lenses, which are close to

4275-443: The normalization of pairs of images. Due to large fields of view required, the time-domain work involves storing and transferring a huge amount of data. This includes data mining techniques, classification, and the handling of heterogeneous data. The importance of time-domain astronomy was recognized in 2018 by German Astronomical Society by awarding a Karl Schwarzschild Medal to Andrzej Udalski for "pioneering contribution to

4350-489: The observer. If the source star is a binary star , then it too will have additional relative motion, which can also cause detectable changes in the light curve. This effect is known as Xallarap (parallax spelled backwards). If the lensing object is a star with a planet orbiting it, this is an extreme example of a binary lens event. If the source crosses a caustic, the deviations from a standard event can be large even for low mass planets. These deviations allow us to infer

4425-435: The ongoing event, hoping to get good coverage of the deviation if it occurs. When the event is over, the light curve is compared to theoretical models to find the physical parameters of the system. The parameters that can be determined directly from this comparison are the mass ratio of the planet to the star, and the ratio of the star-planet angular separation to the Einstein angle. From these ratios, along with assumptions about

4500-503: The only physically significant parameter that can be measured is the Einstein timescale t E {\displaystyle t_{E}} . Since this observable is a degenerate function of the lens mass, distance, and velocity, we cannot determine these physical parameters from a single event. However, in some extreme events, θ E {\displaystyle \theta _{E}} may be measurable while other extreme events can probe an additional parameter:

4575-551: The optical depth toward the LMC to be 1.2×10 , and the optical depth toward the bulge to be 2.43×10 or about 1 in 400,000. Complicating the search is the fact that for every star undergoing microlensing, there are thousands of stars changing in brightness for other reasons (about 2% of the stars in a typical source field are naturally variable stars ) and other transient events (such as novae and supernovae ), and these must be weeded out to find true microlensing events. After

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4650-710: The order of announcement of their discovery) Time-domain astronomy Time-domain astronomy is the study of how astronomical objects change with time. Said to have begun with Galileo's Letters on Sunspots , the field has now naturally expanded to encompass variable objects beyond the Solar System . Temporal variation may originate from movement of the source or changes in the object itself. Common targets include novae , supernovae , pulsating stars , flare stars , blazars and active galactic nuclei . Optical time domain surveys include OGLE , HAT-South , PanSTARRS , SkyMapper , ASAS , WASP , CRTS , GOTO , and

4725-408: The physical parameters of the lens can be solved yielding the lens mass, parallax, and proper motion as In a typical microlensing event, the light curve is well fit by assuming that the source is a point, the lens is a single point mass, and the lens is moving in a straight line: the point source-point lens approximation. In these events, the only physically significant parameter that can be measured

4800-550: The simultaneous effect of many stars. Bohdan Paczyński first used the term "microlensing" to describe this phenomenon. This type of microlensing is difficult to identify because of the intrinsic variability of quasars, but in 1989 Mike Irwin et al. published detection of microlensing of one of the four images in the " Einstein Cross " quasar in Huchra's Lens . In 1986, Paczyński proposed using microlensing to look for dark matter in

4875-480: The size of the Einstein ring in the plane of the observer, known as the Projected Einstein radius : r ~ E {\displaystyle {\tilde {r}}_{E}} . This parameter describes how the event will appear to be different from two observers at different locations, such as a satellite observer. The projected Einstein radius is related to the physical parameters of

4950-408: The source gets even closer in the sky to the lens position, the two images become symmetrical and equal in brightness, and the centroid will again be very close to the true position of the source. When alignment is perfect, the centroid is exactly at the same position as the source (and the lens). In this case, there will not be two images but an Einstein ring around the lens. In practice, because

5025-418: The source star are magnified at different rates at different times during the event. These events can thus be used to study the limb darkening of the source star. If the lens is a binary star with separation of roughly the Einstein radius, the magnification pattern is more complex than in the single star lenses. In this case, there are typically three images when the lens is distant from the source, but there

5100-434: The star. A correct prediction of the concomitant brightening of the source, the basis for microlensing, was published in 1936 by Einstein. Because of the unlikely alignment required, he concluded that "there is no great chance of observing this phenomenon". Gravitational lensing's modern theoretical framework was established with works by Yu Klimov (1963), Sidney Liebes (1964), and Sjur Refsdal (1964). Gravitational lensing

5175-453: The structure of the source and its limb darkening. In principle, the Einstein parallax can be measured by having two observers simultaneously observe the event from different locations, e.g., from the Earth and from a distant spacecraft. The difference in amplification observed by the two observers yields the component of π → E {\displaystyle {\vec {\pi }}_{E}} perpendicular to

5250-440: The survey programs are unable to provide while still searching for new events. The question of how to prioritize events in progress for detailed followup with limited observing resources is very important for microlensing researchers today. Already in his book The Queries (query number 1), expanded from 1704 to 1718, Isaac Newton wondered if a light ray could be deflected by gravity. In 1801, Johann Georg von Soldner calculated

5325-499: The theory of exoplanet microlensing for events from the survey. The MACHO collaboration ended in 1999. Their data refuted the hypothesis that 100% of the dark halo comprises MACHOs, but they found a significant unexplained excess of roughly 20% of the halo mass, which might be due to MACHOs or to lenses within the Large Magellanic Cloud itself. EROS subsequently published even stronger upper limits on MACHOs, and it

5400-575: The time of its announcement in January 2006, the planet OGLE-2005-BLG-390Lb probably had the lowest mass of any known exoplanet orbiting a regular star, with a median at 5.5 times the mass of the Earth and roughly a factor two uncertainty. This record was contested in 2007 by Gliese 581 c with a minimal mass of 5 Earth masses, and since 2009 Gliese 581 e is the lightest known "regular" exoplanet, with minimum 1.9 Earth masses. In October 2017, OGLE-2016-BLG-1190Lb , an extremely massive exoplanet (or possibly

5475-720: The timescale of minutes to decades. Variability studied can be intrinsic , including periodic or semi-regular pulsating stars , young stellar objects , stars with outbursts , asteroseismology studies; or extrinsic , which results from eclipses (in binary stars , planetary transits ), stellar rotation (in pulsars , spotted stars), or gravitational microlensing events . Modern time-domain astronomy surveys often uses robotic telescopes , automatic classification of transient events, and rapid notification of interested people. Blink comparators have long been used to detect differences between two photographic plates, and image subtraction became more used when digital photography eased

5550-422: The true position of the source and the other will be very close to the lens on its left side, and very small or dim. In this case, the centroid is practically in the same position as the source. If the sky position of the source is close to that of the lens and on the right, the main image will be a bit further to the right of the true source position, and the centroid will be to the right of the true position. But as

5625-426: Was first observed in 1979, in the form of a quasar lensed by a foreground galaxy. That same year Kyongae Chang and Sjur Refsdal showed that individual stars in the lens galaxy could act as smaller lenses within the main lens, causing the source quasar's images to fluctuate on a timescale of months, also known as Chang–Refsdal lens . Peter J. Young then appreciated that the analysis needed to be extended to allow for

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