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Foundations of standard theory of microlensing are described, namely we consider microlensing stars in Galactic bulge, the Magellanic Clouds or other nearby galaxies. We suppose that gravitational microlenses lie between an Earth observer and these stars. Criteria of an identification of microlensing events are discussed. We also consider such micr...
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Foundations of standard microlensing theory are discussesd as applied to stars in the Galactic bulge, Magellanic Clouds or other nearby galaxies and gravitational microlenses assumed to lie in-between these stars and the terrestrial observer. In contrast to the review article by Gurevich et al. [48], microlensing by compact objects is mainly consid...
Foundations of standard theory of microlensing are described, namely we consider microlensing stars in Galactic bulge, the Magellanic Clouds or other nearby galaxies. We suppose that gravitational microlenses lie between an Earth observer and these stars. Criteria of an identification of microlensing events are discussed. We also consider such micr...
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Different regimes of gravitational lensing depend on lens masses and roughly correspond to angular distance between images. If a gravitational lens has a typical stellar mass, this regime is named a microlensing because a typical angular distance between images is about microarcseconds in the case when sources and lenses are located at cosmological distances. An angular distance depends on a lens mass as a square root and therefore, if a lens has a typical Earth-like planet mass of 10-6M⊙, such a regime is called nanolensing. Thus, generally speaking, one can call a regime with a planet mass lens a nanolensing (independently on lens and source locations). So, one can name searches for planets with gravitational lens method a gravitational nanolensing. There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, pulsar timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques if we are interested to find Earth-like planets at distances about a few astronomical units from the host star.
Gravitational lensing is based on the gravitational light bending effect. It can be visualized as if a gravitating body attracts photons. In the simplest case of a point-like gravitational lens, the angular distances between images are on the order of the Einstein +- Chwolson diameter, which is proportional to the square root of the lens mass. At cosmological distances between objects and the typical mass of the lensing galaxy, the angular distance between images is on the order of a few arcseconds. The corresponding characteristic time of microlensing is usually determined as the half-time it takes for the lens to cross the Einstein +- Chwolson cone. The searches for microlensing events are closely related to the question of dark matter (DM). The gravitational microlensing of a star by another star produces a symmetric and achromatic light curve, which is the main signature of the phenomenon. he most effective method of discovering exoplanets is based on radial velocity measurements using the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph.
Foundations of standard theory of microlensing are described, namely we consider microlensing stars in Galactic bulge, the Magellanic Clouds or other nearby galaxies. We suppose that gravitational microlenses lie between an Earth observer and these stars. Criteria of an identification of microlensing events are discussed. We also consider such microlensing events which do not satisfy these criteria (non-symmetrical light curves, chromatic effects, polarization effects). We describe results of MACHO collaboration observations towards the Large Magellanic Cloud (LMC) and the Galactic bulge. Results of EROS observations towards the LMC and OGLE observations towards the Galactic bulge are also presented. Future microlensing searches are discussed.
According to a revised schedule of the Russian Space Agency, in October
2008 the 10 m space telescope RadioAstron will be launched in a high
eccentric orbit around the Earth. Acting together with ground based
radio telescopes, the VLBI interferometer with a ground-space arm will
operate. The interferometer will have extraordinary angular resolution
of a few microarcsecond (μas) at the shortest wavelength (1.35 cm).
Since typical angular scales for gravitational microlensing are at the
μas level for cosmological locations of sources and microlenses, in
principle there is a chance to resolve microimages and (or) at least,
detect astrometrical shift of bright point like images. In particular,
gravitationally lensed systems, such as B1600+434, where in radio band a
signature of microlensing is found, look suitable for direct
observations of microlensing, since microlensing with the RadioAstron
interferometer may be detected in the future (considering its high
angular resolution and a relatively high sensitivity and assuming a
ground support by the advanced radio telescopes).
In the review we discuss possible studies of GR phenomena such as
gravitational microlensing and shadow analysis with the forthcoming
RadioAstron space mission. It is well-known that gravitational lensing
is a powerful tool in the investigation of the distribution of matter,
including that of dark matter (DM). Typical angular distances between
images and typical time scales depend on the gravitational lens masses.
For the microlensing, angular distances between images or typical
astrometric shifts are about 10^{-5}- 10^{-6} {as}^{1}. Such an angular
resolution will be reached with the space-ground VLBI interferometer,
Radioastron. The basic targets for microlensing searches should be
bright point-like radio sources at cosmological distances. In this case,
an analysis of their variability and a reliable determination of
microlensing could lead to an estimation of their cosmological mass
density. Moreover, one could not exclude the possibility that
non-baryonic dark matter could also form microlenses if the
corresponding optical depth were high enough. It is known that in
gravitationally lensed systems, the probability (the optical depth) to
observe microlensing is relatively high; therefore, for example, such
gravitationally lensed objects, like CLASS gravitational lens B1600+434,
appear the most suitable to detect astrometric microlensing, since
features of photometric microlensing have been detected in these
objects. However, to directly resolve these images and to directly
detect the apparent motion of the knots, the Radioastron sensitivity
would have to be improved, since the estimated flux density is below the
sensitivity threshold, alternatively, they may be observed by
increasing the integration time, assuming that a radio source has a
typical core - jet structure and microlensing phenomena are caused by
the superluminal apparent motions of knots. In the case of a
confirmation (or a disproval) of claims about microlensing in
gravitational lens systems, one can speculate about the microlens
contribution to the gravitational lens mass. Astrometric microlensing
due to Galactic Macho's action is not very important because of low
optical depths and long typical time scales. Therefore, the launch of
the space interferometer Radioastron will give excellent new facilities
to investigate microlensing in the radio band, allowing the possibility
not only to resolve microimages but also to observe astrometric
microlensing. Shadows around supermassive black holes can be detected
with the RadioAstron space interferometer.
It is well-known that gravitational lensing is a powerful tool in the investigation of the distribution of matter, including that of dark matter (DM). Typical angular dis-tances between images and typical time scales depend on the gravitational lens masses. For the case of microlensing, angular distances between images or typical astrometric shifts are about 10 −5 − 10 −6 arcsec. Such an angular resolution will be reached with the space–ground VLBI interferometer, Radioastron. The basic targets for microlensing searches should be bright point-like radio sources at cosmological distances. In this case, an analysis of their variability and a solid determination of microlensing could lead to an estimation of their cosmological mass density. Moreover, one could not exclude the pos-sibility that non-baryonic dark matter could also form microlenses if the corresponding optical depth were high enough. It is known that in gravitationally lensed systems the probability (the optical depth) of observing microlensing is relatively high. Therefore, for example, gravitationally lensed objects, like the CLASS gravitational lens B1600+434, appear to be most suitable to detect astrometric microlensing, since features of photo-metric microlensing have been detected in these objects. However, to directly resolve these images and to directly detect the apparent motion of the knots, the Radioastron sensitivity would have to be improved, since the estimated flux density is below the sensitivity threshold. Alternatively, they may be observed by increasing an integration time, assuming that a radio source has a typical core–jet structure and microlensing phenomena are caused by the superluminal apparent motions of knots. In the case of a confirmation (or a negation) of claims about microlensing in gravitational lens systems, one can speculate about the microlens contribution to the gravitational lens mass. Astro-metric microlensing due Galactic MACHOs is not very important because of low optical depths and long typical time scales. Therefore, the launch of the space interferometer Radioastron will enable the investigation of microlensing in the radio band, giving rise 1055 Int.
Different regimes of gravitational lensing depend on lens masses and roughly correspond to angular distance between images. If a gravitational lens has a typical stel-lar mass, this regime is named microlensing because the typical angular distance between images is about microarcseconds in the case for sources and lenses at cosmological dis-tances. The angular distance depends on as a squared root of lens mass and therefore, for Earth-like planet mass lens (10 −6 M), such a regime is called nanolensing. So, one can name searches for exoplanets with gravitational lens method as gravitational nanolensing. There are different methods for finding exoplanets such as radial spectral shifts, astrometri-cal measurements, transits, timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potentiality of detecting Earth-like planets at distances about a few astronomical units from their host star.
Many exotic astronomical objects have been introduced. Usually the objects have masses, therefore they may act as gravitational
lenses. We briefly discuss gravitational lensing with cosmic strings. As is well-known, dark matter is one of the most important
components of the Universe. Recent computer simulations indicate that dark matter may form clumps. We review gravitational
lensing (including microlensing) for the clumps.
KeywordsGravitational lensing-Microlensing techniques (astronomy)-Gravitational lenses and luminous arcs-Dark matter
Gravitational lensing is a powerful tool for studies of the distribution of matter (including dark matter) in the Universe.
The characteristic angular separation of images and characteristic variability time scale depend on the characteristic masses
of the gravitational lenses. The construction of the RADIOASTRON space interferometer in the coming years will provide qualitatively
new conditions for investigations of microlensing of distant quasars in the radio, since it will become possible not only
to resolve individual microimages of these quasars, but also to observe astrometric microlensing (i.e., the shift of an image
of a distant quasar, sometimes called weak microlensing). Astrometric microlensing by compact objects in the Galactic halo
is not a very significant effect for the RADIOASTRON interferometer, since the probability of such events is low and the characteristic
time scales involved appreciably exceed the proposed operational lifetime of the interferometer.
Foundations of standard theory of microlensing are described, namely we consider microlensing stars in Galactic bulge, the Magellanic Clouds or other nearby galaxies. We suppose that gravitational microlenses lie between an Earth observer and these stars. Criteria of an identification of microlensing events are discussed. We also consider such microlensing events which do not satisfy these criteria (non-symmetrical light curves, chromatic effects, polarization effects). We describe results of MACHO collaboration observations towards the Large Magellanic Cloud (LMC) and the Galactic bulge. Results of EROS observations towards the LMC and OGLE observations towards the Galactic bulge are also presented. Future microlensing searches are discussed.
