Sharif Digital Repository

The finite-size effect in gravitational microlensing provides a possibility to measure the variation of surface brightness of stars from centre to limb which is called limb darkening profile. Measuring limb darkening profile provides information about star atmosphere. It is also useful in other fields of astrophysics. In this thesis we develop a numerical method to recover limb darkening of source stars from single lens microlensing light curves with finite source effect in which the lens transits the source disk. We use the Finite Element Method (FEM) as an inversion tool for discretization and inversion of the finite source effect integral equation. This method makes no explicit assumption... 

In this thesis, we investigate the imprint of gravitational microlensing effects on the light curves of Gamma-ray burst (GRBs) to estimate compact object mass in the form of black holes in the Universe. We assume a point mass model of the gravitational lens and a geometry in which the source−lens are at the same direction that manifests in the GRB light curve as superimposed peaks by the time delay between the two images and searches the Fermi/GBM catalog. According to the sensitivity of our detection method, we consider microlensing events with a time delay on the order of 10 s due to point-mass lenses in the range of 〖10〗^5- 〖10〗^7 M_(⊙ ). We found one microlensed GRB candidate out of... 

Gravitational Microlensing has been used as a powerful tool for astrophysical studies and exoplanet detections. In the gravitational microlensing, we have two images with negative and positive parities. The negative-parity image is a fainter image and is produced at a closer angular separation with respect to the lens star. In the case of a red-giant lens star and large impact parameter of lensing, this image can be eclipsed by the lens star. The result would be dimming the flux receiving from the combination of the source and the lens stars and the light curve resembles to an eclipsing binary system. In this work, we introduce this phenomenon and propose an observational procedure for... 

When in a binary system one of the stars passes behind another star eclipsing binary will occur and light’s intensity will decrease. Now with assumption that one of its component be Black Hole When another star passes behind of it the star’s light is amplified by Microlensing.With detection of this amplification we can find black hole’s mass. Of course in this phenomenon the degeneracy will be broken completely and we can obtain all of the quantity. From another specification of this event we can call the shortness and repetitive of Einestein’time .The Einestein’s time is approximately a few hours and depending on system’s period this event will be repeated in specific time.Finally Black... 

In this thesis, we investigate two subjects in gravitational microlensing. In first part we study wave optics effect in light curve of a binary system. Gravitational lensing occurs when light deflects in a gravitational field. This deflection produces multi-images from real one. In microlensing regime, images can not be detected separately and gravitational lensing appears as magnifying source’s flux . Microlensing has been studied mostly in geometric optics regime. At larger wavelengths, mostly at radio wavelengths, we expect to see the effects of wave optics in microlensing. Stars at radio wave are faint and to detect them we need a powerful radio telescope. In the near future, SKA (Square... 

At the beginning of the 20th century, Albert Einstein introduced the equations of general relativity, opening a new avenue for investigating various phenomena. One of the predictions of these equations, gravitational lensing, was observed by Eddington during his lifetime, confirming the theory's ability to describe nature. Gravitational lensing can be categorized into three types: strong lensing, weak lensing, and microlensing. The current focus of research is on observing the separation phenomenon in gravitational microlensing. The observation of gravitational microlensing provides valuable insights into the composition of celestial bodies in the universe. Calculating the mass of different... 

Gravitational microlensing with the binary lenses are one the tools for detection of exoplanets. Due to the degeneracy nature of the solutions for the observed light curves additional features such as parallax and finite-size effects help to break the degeneracy. Here in this work we introduce the application of frequency shift effect in the microlensing observations where for the single lens, it has been studied in [5] .In this work our purpose is to study the frequency shifts in binary lenses. Therefore, after reviewing the frequency shift in single gravitational lens systems, we will study this phenomenon in binary systems.In binary lenses, which are mainly used to detect extrasolar... 

In this thesis first we will review gravitational lensing and microlensing, then we will introduce all direct and indirect methods for detecting exo-planets. gravitational microlensing is one of these methods in detecting exo-planets. In the second part first we will explain a powerful numerical method for calculating magnification, position and the shape of images by combining of tree code and inverse rays hooting algorithm. Then we simulated the caustic crossing of Hot Jupiters using that algorithm. Finally with performing Monte-Carlo simulation we will obtain the detection efficiency for detecting this class of exo-planets during caustic crossing and we will estimate the number of these... 

Discovery of the effect of a massive object on the light path, reaching its exact formalism through general relativity and utilizing this phenomenon as a natural lens, opened a new window in astronomical research. Gravitational lensing occurs in three different ways; strong lensing, weak lensing, and microlensing which we focus on the third one. Gravitational microlensing is useful in studying dark matter candidates, discovering exoplanets, finding the surface properties of stars, and calculating the mass distribution of galactic objects. From a typical microlensing event, Einstein crossing time is obtained which is the time scale of source star passing by the lens. This is the only... 

Gravitational microlensing is one of the experimental tests to confirm the theory of general relativity. In this phenomenon, the light of the source star is deflected by the gravitational field of the foreground star which plays the role of the lens. As a result of this deviation, images of source star are formed on the sky plane. The observer records the light of the source star as a variable with time. The gravitational microlensing illustrates the possibility to discover extrasolar planets, study the atmosphere of stars, and probe the nature of Dark matter. In this thesis, at first, we introduce the basic concepts of gravitational microlensing and its history. Then by simulation of... 

Gravitational Microlensing is one of the most powerful methods of detecting very low mass objects like Exoplanets and Brown dwarfs. The most important parameter that we can extract from a microlensing event is Einstein radius crossing time t_E. In this thesis, with performing Monte-Carlo simulation, we will obtain t_E distribution for brown dwarf population. Then we will show that this population can be a good candidate for very short microlensing events with t_E<2 days. The data set used in this analysis was taken in 2006 and 2007 seasons by the MOA-II survey, with the 1.8-m MOA-II telescope located at the Mt. John University Observatory, New Zealand  

We investigate the possibility of exoplanet detection orbiting source stars in microlensing events through the Spitzer and WFIRST follow-up observations.We perform a Monto Carlo simulation on the detection rate of exoplanets via microlensing, assuming that each source star has at least one exoplanet. The exoplanet can reflect part of the light from the parent star or emit internal thermal radiation. In this new detection channel, we use microlensing as an amplifier to magnify the reflection light from the planet.In the literature, this mode of detecting exoplanets has been investigated much less than the usual mode in which the exoplanets are considered as one companion in binary lens...