Polarimetric microlensing of circumstellar disks

TL;DR

Polarimetric observations of microlensing events can reveal detailed geometrical properties of circumstellar disks around stars in the Galactic bulge, providing complementary information to photometry.

Contribution

This work demonstrates that polarimetry can constrain disk geometry and improve detection of circumstellar disks during microlensing events.

Findings

Polarimetry helps constrain disk inner radius and orientation.

Polarimetric time scales are generally longer than photometric ones.

Approximately 4 disks can be detected or characterized in 10 years of monitoring.

Abstract

We study the benefits of polarimetry observations of microlensing events to detect and characterize circumstellar disks around the microlensed stars located at the Galactic bulge. These disks which are unresolvable from their host stars make a net polarization effect due to their projected elliptical shapes. Gravitational microlensing can magnify these signals and make them be resolved. The main aim of this work is to determine what extra information about these disks can be extracted from polarimetry observations of microlensing events in addition to those given by photometry ones. Hot disks which are closer to their host stars are more likely to be detected by microlensing, owing to more contributions in the total flux. By considering this kind of disks, we show that although the polarimetric efficiency for detecting disks is similar to the photometric observation, but polarimetry observations can help to constraint the disk geometrical parameters e.g. the disk inner radius and the lens trajectory with respect to the disk semimajor axis. On the other hand, the time scale of polarimetric curves of these microlensing events generally increases while their photometric time scale does not change. By performing a Monte Carlo simulation, we show that almost 4 optically-thin disks around the Galactic bulge sources are detected (or even characterized) through photometry (or polarimetry) observations of high-magnification microlensing events during 10 years monitoring of 150 million objects.