Sharif Digital Repository

The detection of signals from a possible extrasolar technological civilization is one of the most challenging efforts of science. In this work, we propose using natural telescopes made of single or binary gravitational lensing systems to magnify leakage of electromagnetic signals from a remote planet that harbors Extraterrestrial Intelligent (ETI) technology. Currently, gravitational microlensing surveys are monitoring a large area of the Galactic bulge to search for microlensing events, finding more than 2000 events per year. These lenses are capable of playing the role of natural telescopes, and, in some instances, they can magnify radio band signals from planets orbiting around the source... 

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... 

In this paper, we review the astrophysical application of gravitational microlensing. After introducing the history of gravitational lensing, we present the key equations and concept of microlensing. The most frequent microlensing events are single-lens events and historically it has been used for searching dark matter in the form of compact astrophysical halo objects in the Galactic halo. We discuss about the degeneracy problem in the parameters of lens and perturbation effects that can partially break the degeneracy between the lens parameters. The rest of paper is about the astrophysical applications of microlensing. One of the important applications is in the stellar physics by probing... 

In this work, we study the interaction of the electromagnetic wave (EW) from a distant quasar with the gravitational wave (GW) sourced by the binary stars.While in the regime of geometric optics the light bending due to this interaction is negligible, we show that the phase shifting on the wavefront of an EW can produce the diffraction pattern on the observer plane. The diffraction of the light (with the wavelength of λe) by the gravitational wave playing the role of gravitational grating (with the wavelength of λg) has the diffraction angle of Δβ ~ λe/λg. The relative motion of the observer, the source of gravitational wave, and the quasar results in a relative motion of the observer... 

The relative transverse velocity of a lens with respect to the source star in gravitational lensing results in a frequency shift in the light rays passing by a lens. We propose using this relativistic effect for measuring the relative velocity of the lens with respect to the source star in gravitational microlensing. High precision spectrographs with the accuracy of detecting the relative frequency shift in the order of 10-11 will enable us to measure this effect in the microlensing events. The spectrographs such as ESPRESSO are going to be used for detecting exoplanets with the accuracy of the radial velocity of 0.1 m/s. This kind of instrument can be used in follow-up observations of the... 

The main aim of microlensing experiments is to evaluate the mean mass of massive compact I halo objects (MACHOs) and the mass fraction of the Galactic halo made by this type of dark matter. Statistical analysis shows that by considering a Dirac-Delta mass function (MF) for the MACHOs, their mean mass is about that of a white dwarf star. This result is, however, in discrepancy with other observations such as those of non-observed expected white dwarfs in the Galactic halo which give rise to metal abundance, polluting the interstellar medium by their evolution. Here we use the hypothesis of the spatially varying MF of MACHOs, proposed by Kerins and Evans to interpret microlensing events. In... 

MOG as a modified gravity theory is designed to be replaced with dark matter. In this theory, in addition to the metric tensor, a massive vector is a gravity field where each particle has a charge proportional to the inertial mass and couples to the vector field through the four-velocity of a particle. In this work, we present the Hamiltonian formalism for the dynamics of particles in this theory. The advantage of Hamiltonian formalism is a better understanding and analysing the dynamics of massive and massless particles. The massive particles deviate from the geodesics of space-time and photons follow the geodesics. We also study the dynamics of particles in the Newtonian and post-Newtonian... 

After a decade of gravitational microlensing experiments, 13 to 17 events MACHO group (depending on quality) and two events by EROS have been detected. All of those have been observed in the direction of Large Magellanic Cloud. We use Evans spherically symmetric halo model to study the rate of microlensing events. The expected number of events in this models obtained using EROS and MACHO observational efficiencies. We compare our numbers with the observed events to obtain the fraction of halo that is made by compact objects. It is shown that results derived from the two experiments are in good agreement with each other and MACHOs comprise only a fraction (depending on the model) of Milky Way... 

We investigate relation between density contrast and peculiar velocity in the Newtonian gauge, in general relativistic framework of theory of large scale structure formation in the universe. According to the gauge-invariant property of the energy-momentum tensor in this gauge, the velocity perturbation behaves like the Newtonian peculiar velocity. In this framework, the relation between peculiar velocity and density contrast with respect to the Newtonian-Peebles formula has an extra correction term which is ignorable for the small scale structures. The relativistic correction of peculiar velocity for structures of the extent of hundred mega parsec is about few percents which is smaller than... 

After a decade of gravitational microlensing experiments, a dozen microlensing candidates in the direction of the stars of the Large Magellanic Cloud (LMC) have been detected by the EROS and MACHO groups. Recently it was shown that the distribution of the duration of the observed LMC microlensing events is significantly narrower than what is expected from the standard halo model. In this article we make the same comparison, using non-standard halo models and considering the contribution of non-halo components of the Milky Way such as the disc, spheroid and the LMC itself in the microlensing events. Comparing the theoretical and experimental widths of the distribution of the duration of... 

In this work, we search for signatures of gravitational millilensing in gamma-ray bursts (GRBs) in which the source-lens-observer geometry produces two images that manifest in the GRB light curve as superimposed peaks with identical temporal variability (or echoes), separated by the time delay between the two images. According to the sensitivity of our detection method, we consider millilensing events due to point-mass lenses in the range of 105 - 107 M o˙ at lens redshift about half that of the GRB, with a time delay on the order of 10 s. Current GRB observatories are capable of resolving and constraining this lensing scenario if the above conditions are met. We investigated the Fermi/GBM... 

We develop a general data-driven and template-free method for the extraction of event waveforms in the presence of background noise. Recent gravitational-wave observations provide one of the significant scientific areas requiring data analysis and waveform extraction capability. We use our method to find the waveforms for the reported events from the first, second, and third LIGO observation runs (O1, O2, and O3). Using the instantaneous frequencies derived by the Hilbert transform of the extracted waveforms, we provide the physical time delays between the arrivals of gravitational waves to the detectors. © 2021, The Author(s)  

We develop a general data-driven and template-free method for the extraction of event waveforms in the presence of background noise. Recent gravitational-wave observations provide one of the significant scientific areas requiring data analysis and waveform extraction capability. We use our method to find the waveforms for the reported events from the first, second, and third LIGO observation runs (O1, O2, and O3). Using the instantaneous frequencies derived by the Hilbert transform of the extracted waveforms, we provide the physical time delays between the arrivals of gravitational waves to the detectors. © 2021, The Author(s)  

We investigate the stochasticity in temperature fluctuations in the cosmic microwave background (CMB) radiation data from the Wilkinson Microwave Anisotropy Probe. We show that the angular fluctuation of the temperature is a Markov process with a Markov angular scale, ΘMarkov = 1.01 -0.07+0.09. We characterize the complexity of the CMB fluctuations by means of a Fokker-Planck or Langevin equation and measure the associated Kramers-Moyal coefficients for the fluctuating temperature field T (n̂)and its increment, ∇ T = T(n̂2). Through this method we show that temperature fluctuations in the CMB have fat tails compared to a Gaussian distribution. © 2006 IOP Publishing Ltd and SISSA  

We investigate the statistical anisotropy and Gaussianity of temperature fluctuations of Cosmic Microwave Background (CMB) radiation data from the Wilkinson Microwave Anisotropy Probe survey, using the Multifractal Detrended Fluctuation Analysis, Rescaled Range, and Scaled Windowed Variance methods. Multifractal Detrended Fluctuation Analysis shows that CMB fluctuations has a long-range correlation function with a multifractal behavior. By comparing the shuffled and surrogate series of CMB data, we conclude that the multifractality nature of the temperature fluctuation of CMB radiation is mainly due to the long-range correlations, and the map is consistent with a Gaussian distribution  

The excursion set approach is a framework for estimating how the number density of nonlinear structures in the cosmic web depends on the expansion history of the universe and the nature of gravity. A key part of the approach is the estimation of the first-crossing distribution of a suitably chosen barrier by random walks having correlated steps: The shape of the barrier is determined by the physics of non-linear collapse, and the correlations between steps by the nature of the initial density fluctuation field. We describe analytic and numerical methods for calculating such first up-crossing distributions. While the exact solution can be written formally as an infinite series, we show how to...