Gravitational microlensing

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Gravitational Lensing
Formalism
Weak lensing
Microlensing
Einstein ring
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Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects ranging in mass from 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 lots of light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a tiny fraction 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 a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Einstein in 1915, leads to two distorted unresolved images 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.

Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint. It is thus an ideal technique 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 Objects. Moreover, the microlensing effect is wavelength-independent, allowing to study source objects that emit any kind of electromagnetic radiation.

Microlensing by an isolated object was first detected in 1993. Since then, microlensing has been used to constrain the nature of the dark matter, detect extrasolar planets, 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 a means to find dark objects like brown dwarfs and black holes, study starspots, measure stellar rotation, look for cosmic strings, study globular clusters, and probe quasars.

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[edit] How it works

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 the background source.[1]

Microlensing is caused by the same physical effect as strong lensing and weak lensing, but it is studied using very different observational techniques. In strong and weak lensing, the mass of the lens is large enough (mass of a galaxy or a galaxy cluster) that the displacement of light by the lens can be resolved with a high resolution telescope such as 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 the event. Thus, unlike with strong and weak gravitational lenses, a microlensing event is a transient phenomenon.[2]

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 is known as a light curve. A typical microlensing light curve is shown below:

Typical light curve of gravitational microlensing event (OGLE-2005-BLG-006) with its model fitted (red)

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:

  • Lens mass distribution. If the lens mass is not concentrated in a single point, the light curve can be dramatically different, particularly with caustic-crossing events, which may exhibit strong spikes in the light curve. In microlensing, this can be seen when the lens is a binary star or a planetary system.
  • Finite source size. In extremely bright or quickly-changing microlensing events, like caustic-crossing events, the source star cannot be treated as an infinitesimally small point of light: the size of the star's disk and even limb darkening can mollify extreme features.
  • Parallax. For events lasting for months, the motion of the Earth around the Sun can cause the alignment to change slightly, affecting the light curve.

Most focus is currently on the more unusual microlensing events, especially those that might lead to the discovery of extrasolar planets. Although it has not yet been observed, another way to get more information from microlensing events that may soon be feasible involves measuring the astrometric shifts in the source position during the course of the event[3] and even resolving the separate images with interferometry.[4]

[edit] Observing microlensing

In practice, because 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, 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 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 the optical depth toward the LMC to be 1.2×10-7 or about 1 in 8,000,000,[5] and the optical depth toward the bulge to be 2.43×10-6 or about 1 in 400,000.[6]

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 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 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.

[edit] History

In 1704 Isaac Newton suggested that a light ray could be deflected by gravity. In 1801 Johann Georg von Soldner calculated the amount of deflection of a light ray from a star under Newtonian gravity. In 1915 Einstein correctly predicted the amount of deflection under General Relativity, which was twice this amount. This prediction was validated by a 1919 expedition led by Arthur Eddington, which was a great early success for General Relativity.[7] In 1924 Orest Chwolson found that lensing could produce multiple images of the star. A correct prediction of the concomitant brightening of the source, the basis for microlensing, was published in 1936 by Einstein in the journal Science. 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).[8]

Gravitational lensing 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. 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 in Huchra's Lens.

In 1986, Paczyński proposed using microlensing to look for dark matter in 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[9] and the French EROS[10] collaboration. In 1991 Paczyński suggested that microlensing might be used to find planets, and in 1992 he founded the OGLE microlensing experiment,[11] 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[12] and EROS[13] in 1993, and in the following years, events continued to be detected. 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.[14] EROS subsequently published even stronger upper limits on MACHOs,[15] and it is currently uncertain as to whether there is any halo microlensing excess that could be due to dark matter at all. The SuperMACHO project[16] 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 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)[17] 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, followup projects are underway to study in detail potentially interesting events in progress, primarily with the aim of detecting extrasolar planets. These include PLANET[18] and MicroFUN.[19]

[edit] Mathematics

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 the lens mass M, the distance of the lens dL, and the distance of the source dS:

\theta_E = \sqrt{\frac{4GM}{c^2} \frac{d_S - d_L}{d_S d_L}} (in radians)

For M equal to the mass of the Sun, dL = 4000 parsecs, and dS = 8000 parsecs (typical for a Bulge microlensing event), the Einstein radius is 0.001 arcseconds (1 milliarcsecond). By comparison, ideal Earth-based observations have angular resolution around 0.4 arcseconds, 400 times greater. In addition to being the radius of the Einstein ring, the value of θE pertains to the microlensing event in several ways. 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 θE. Thus the event duration is determined by the time it takes the apparent motion of the lens in the sky to cover an angular distance θE. 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 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 θE. The amplification factor is given in terms of this value:

A(u) = \frac{u^2 + 2}{u \sqrt{u^2 + 4}}

This function has several important properties. A(u) 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, A(u) approaches infinity as the images approach an Einstein ring. For perfect alignment (u = 0), A(u) is theoretically infinite. In practice, 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 with strong lensing, in microlensing u changes significantly in a short period of time. The relevant time scale is called the Einstein time tE, and it's given by the time it takes the lens to traverse an angular distance θE in the sky. For typical microlensing events, tE is on the order of a few days to a few months. The function u(t) is simply determined by the Pythagorean theorem:

u(t) = \sqrt{u_{min}^2 + (\frac{t-t_0}{t_E})^2}

The minimum value of u, called umin, determines the peak brightness of the event.

[edit] Detection of extrasolar planets

Gravitational microlensing of an extrasolar planet
Gravitational microlensing of an extrasolar planet

If the lensing object is a star with a planet orbiting it, then the planet can be detected by its influence on the microlensing event, which is manifest as a deviation from the light curve that would occur if the star alone were acting as a 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. Followup groups then intensively monitor 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 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.[20] As of February 2008, a total of six exoplanets have been detecting in microlensing events, including OGLE-2005-BLG-071,[21] OGLE-2005-BLG-390,[22], OGLE-2005-BLG-169,[23], and two exoplanets in OGLE-2006-BLG-109[24]. Notably, at 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 is now contested by Gliese 581 c with a minimal mass of 5 times the mass of the Earth in April 2007, where either of these has a fair chance to be the less massive, but OGLE-2005-BLG-390Lb still being the frontrunner. With its higher surface temperature, Gliese 581 c however appears to be the smaller planet.

This method of detecting extrasolar planets has both advantages and disadvantages compared 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. One disadvantage is that followup of the lens system is very difficult after the event has ended, because it takes a long time for the lens and the source to be sufficiently separated to resolve them separately.

[edit] 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. There are current proposals to build new specialized microlensing satellites, or to use other satellites to study microlensing.

[edit] Search collaborations

[edit] Follow-up collaborations

[edit] Andromeda galaxy pixel lensing collaborations

[edit] Proposed satellite experiments

[edit] References

  1. ^ Refsdal 1964. Monthly Notices of the Royal Astronomical Society. adsabs.harvard.edu
  2. ^ Paczynski 1986. Astrophysical Journal [1]
  3. ^ adsabs.harvard.edu
  4. ^ adsabs.harvard.edu
  5. ^ Alcock et al. 2000. Astrophysical Journal [2]
  6. ^ Alcock et al. 2000. Astrophysical Journal. [3]
  7. ^ Schneider, Ehlers, and Falco. Gravitational Lenses. 1992.
  8. ^ Wambsganss 2006 [4]
  9. ^ mcmaster.ca
  10. ^ eros.in2p3.fr
  11. ^ bulge.astro.princeton.edu
  12. ^ Alcock et al. 1993. Nature. [5]
  13. ^ Aubourg et al. 1993. Nature. [6]
  14. ^ Alcock et al. 2000. Astrophysical Journal. [7]
  15. ^ Tisserand et al. 2007. Astronomy and Astrophysics Journal [8]
  16. ^ http://www.ctio.noao.edu/supermacho/
  17. ^ phys.canterbury.ac.nz
  18. ^ planet.iap.fr
  19. ^ astronomy.ohio-state.edu
  20. ^ Bond et al. 2004. Astrophysical Journal [9]
  21. ^ Udalski et al. 2005. Astrophysical Journal. [10]
  22. ^ OGLE website [11]
  23. ^ Gould et al. 2006. Astrophysical Journal. [12]
  24. ^ Gaudi et al. 2008. Science. [13]

[edit] External links