In the first section (introduction), the writer tried to give a brief historical review of the theoretical origin of gravitational lensing followed by the history of investigation about this phenomenon near single stars which is called Micro lensing only because of weakness of the effect of a low mass object (single star) on the nearby space-time curvature compared to more massive objects (galaxies or galaxy groups). He also mentioned the early suggestion about using microlensing for detecting planetary companions around stars and then he goes through observational issues like observations which led to first record of microlensing and programs for planet search using this method and the evolution of this method from the beginning until present.
In the 2nd section (Foundational Concepts and Equations )a brief and simple review of the theory of microlensing is given. I’m not going through it because of lack of space in this one page report.
Section 3 (Practice of Microlensing) is the main section in this brief report. A microlensing event is defined as apparent relative motion between lens and the source which gives rise to time-variable magnification of the source. Assuming rectilinear motion between the source and the lens we can write apparent angular separation of the source and the lens in the units of Einstein’s radius and time. The figure below shows magnification as a function of time for a single lens object with.
The measurable quantity is of course not the magnification itself but the photometered flux of the target as a function of time which is given by,
.
The lightcurve of a single lens object can be fit by five parameters and it’s important to note that several of these parameters are highly degenerate. There are four gross observables in the lightcurve . As a result of these degeneracies, when fitting to data it is often useful to employ an alternate parameterization of the single-lens model that is more directly tied to these gross observables, in order to avoid strong covariances between the model parameters. In practice, several different observatories using several different filters typically contribute data to any given observed microlensing event. Since the flux of the source and blend will vary depending on the specific filter, and furthermore different observatories may have different resolutions and thus different amounts of blended light, one must allow for a different source and blend flux for each filter/observatory combination. It’s obvious that the number of observed parameters increases with the number of the independent observations of an event. Using this data set, one can search for finding the best values of fitting parameters . For more details on the method used for finding these parameters, check the original document.
This simple model shows the general behavior of single lens/source event, which can be perturbed by several facts in reality. The most common and interesting anomaly is when the lens is not a single object. With this type of anomaly we can infer the presence of a companion near the lens star and also find basic properties of the companion, like mass ratio and distance. The other sources of perturbation are:
· Finite source effect
· Parallax
· Xallarap
· Orbital motion
· Binary sources
For more details of each effect see pages 18 and 19 in the article.
The next subsection discusses about how to infer the properties of exoplanetary system from the lightcurve. I think summarizing this subsection in this small report could cause some misconceptions for the readers. Interested readers can follow the details in the original text.
Practical aspects of current microlensing searches is reviewed in the last subsection. In section 4 readers can find some interesting features of microlensing methods like:
· Peak Sensitivity Beyond the Snow Line
· Sensitivity to Low-mass Planets
· Sensitivity to Long-Period and Free-Floating Planets
· Sensitivity to Planets Orbiting a Wide Range of Host Stars
· Sensitivity to Planets Throughout the Galaxy
· Sensitivity to Multiple-Planet Systems
· Sensitivity to Moons of Exoplanets
The last section of the article gives us some of the recent highlights in this field of research.
In the end look at the pictures below to understand how a planetary companion can disturb the light curve of a normal microlensing event.
Pictures from OLGE project.Click on the pictures to see the animated version!!!
Look also:
REFERENCES:
- The main source of this post is a review article “Exoplanetary Microlensing” written by B. Scott Gaudi (http://arxiv.org/abs/1002.0332v2), published on Feb. 3rd 2010 on “arxiv”. The aim of this paper is to give an introduction to the discovery and characterization of exoplanet with gravitational microlensing.
- Planetary Microlensing OGLE 2003-BLG-235/MOA 2003-BLG-53 FIRST DETECTION of an EXTRASOLAR PLANET with MICROLENSING
congartulations,be sucessful..!
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