Photometric, astrometric and polarimetric observations of gravitational microlensing events

Abstract

The gravitational microlensing as a unique astrophysical tool can be used for studying the atmosphere of stars thousands of parsec far from us. This capability results from the bending of light rays in the gravitational field of a lens which can magnify the light of a background source star during the lensing. Moreover, one of properties of this light bending is that the circular symmetry of the source is broken by producing distorted images at either side of the lens position. This property makes the possibility of the observation of the polarization and the light centroid shift of images. Assigning vectors for these two parameters, they are perpendicular to each other in the simple and binary microlensing events, except in the fold singularities. In this work, we investigate the advantages of polarimetric and astrometric observations during microlensing events for (i) studying the surface of the source star and spots on it and (ii) determining the trajectory of source stars with respect to the lens. Finally we analyze the largest sample of microlensing events from the OGLE catalog and show that for almost ∼ 4.3% of events in the direction of the Galactic bulge, the polarization signals with large telescopes would be observable.

Polarimetric microlensing of circumstellar discs

“…Ingrosso et al (2012Ingrosso et al ( ,2015 evaluated the expected polarization signals for a set of reported high-magnification single-lens and exo-planetary microlensing events as well as OGLE-III events towards the Galactic bulge. Recently, Sajadian and Rahvar (2015) noticed that there is an orthogonal relation between the polarization and astrometric shift of source star position in the simple and binary microlensing events except in the fold singularities and investigated the advantages of this correlation for studying the surface of a source star and spots on it.…”

“…Whereas, the polarization signal of the source is always normal to the lens-source connection line (see e.g. Sajadian and Rahvar 2015). When the lens is entering into an inclined disk normal to the disk semimajor axis (ξ = 90 • ), the polarizations of the disk and source are normal to each other.…”

“…The maximum polarimetric precision which is achievable by this instrument is 0.1 per cent by taking one hour exposure time from a source star brighter than 14.5 mag (Ingrosso et al 2015) which is equivalent to S/R ∼ 34000. Our definition of S/N can be found in Sajadian (2015b). We consider the following relation between S/N and the polarimetric precision of the FORS2 polarimeter (Schemid et al 2002):…”

“…On the other hand, polarimetric microlensing can probe the surface and the atmosphere of the source stars by magnify the small polarization signals from the anomalies on the surface of stars such as stellar spots and magnetic fields to be detected (Sajadian & Rahvar 2015;Sajadian 2015b). Another anomaly in the star atmosphere that makes a net polarization is the existence of a circumstellar disk around the source.…”

Parallax and Orbital Effects in Astrometric Microlensing With Binary Sources

“…When the lens is a binary system (see e.g. , Safizadeh et al 1999, Bozza 2001, Hideki 2002, Sajadian & Rahvar 2015, the number and the position of the images differ from those of the single lens case and the astrometric signal trajectory and the deviation varies depending on on the binary system parameters (i.e., the mass ratio and the component separation).…”