arXiv subjects

Ahmad Mehrabi

Publications and source records attributed to Ahmad Mehrabi.

21 records · Page 2Linked to original sources

arXiv2015

On the possibility of magnetic field detection on a microlensed source star

In a microlensing event, a large magnification occurs at caustic crossing and provides an opportunity to obtain a stronger signal associated with the object. In this paper we study the possibility of magnetic field detection in a microlensing event through the Zeeman effect. We follow the prescription introduced by \citep{Robinson:1980} which analyses the spectrum of a star in a Fourier space to deconvolve other broadening mechanism from the actual Zeeman effect. First we study magnification contrast between source and spot in terms of spot size and then consider two distinct strategies using modern spectrographs and perform a Monte Carlo simulation to find the detection efficiency in magnification-magnetic field plan. The spectral resolution and signal to noise ratio in each strategies, specify suitable places in this plane to detect a magnetic field. Apart from complexity of magnetic field on a star, we consider a simple model and propose a fantastic method to detect magnetic field using spectrum of source stars at caustic crossing.

astro-ph.SR↗

arXiv2014

Can observational growth rate data favour the clustering dark energy models?

Under the commonly used assumption that clumped objects can be well described by a spherical top-hat matter density profile, we investigate the evolution of the cosmic growth index in clustering dark energy (CDE) scenarios on sub-horizon scales. We show that the evolution of the growth index $γ(z)$ strongly depends on the equation-of-state (EoS) parameter and on the clustering properties of the dark energy (DE) component. Performing a $χ^2$ analysis, we show that CDE models have a better fit to observational growth rate data points with respect to the concordance $Λ$CDM model. We finally determine $γ(z)$ using an exponential parametrization and demonstrate that the growth index in CDE models presents large variations with cosmic redshift. In particular it is smaller (larger) than the theoretical value for the $Λ$CDM model, $γ_Λ\simeq0.55$, in the recent past (at the present time).

astro-ph.CO↗

arXiv2013

Studying wave optics in the light curve of exoplanet microlensing

We study the wave optics features of gravitational microlensing by a binary lens composed of a planet and a parent star. In this system, the source star near the caustic line produces a pair of images in which they can play the role of secondary sources for the observer. This optical system is similar to the Young double-slit experiment. The coherent wave fronts from a source on the lens plane can form diffraction pattern on the observer plane. This diffraction pattern has two modes from the close- and wide-pair images. From the observational point of view, we study the possibility of detecting this effect through the Square Kilometer Array (SKA) project in the resonance and high magnification channels of binary lensing. While the red giant sources do not seem satisfy the spatial coherency condition, during the caustic crossing, a small part of source traversing the caustic line can produce coherent pair images. Observations of wave optics effect in the longer wavelengths accompanied by optical observations of a microlensing event provide extra information from the parameter space of the planet. These observations can provide a new basis for study of exoplanets.

astro-ph.EP↗