Gravitational Microlensing as a probe of the Electron Scattering Region in Q2237+0305

TL;DR

This paper explores how gravitational microlensing can be used to determine the size and orientation of the Electron Scattering Region in quasars by analyzing polarized light variations, providing a new method to probe AGN structures.

Contribution

It introduces a novel approach using microlensing-induced polarization variations to constrain the geometry of the Electron Scattering Region in AGNs.

Findings

Correlated polarization light curves reveal ESR size and orientation.

The method is robust against lens shear and convergence effects.

Polarization-based measurements can be made on shorter timescales.

Abstract

Recent observations have provided strong evidence for the presence of an Electron Scattering Region (ESR) within the central regions of AGNs. This is responsible for reprocessing emission from the accretion disk into polarised radiation. The geometry of this scattering region is, however, poorly constrained. In this paper, we consider the influence of gravitational microlensing on polarised emission from the ESR in the quadruply imaged quasar, Q2237+0305, demonstrating how correlated features in the resultant light curve variations can determine both the size and orientation of the scattering region. This signal is due to differential magnification between perpendicularly polarised views of the ESR, and is clearest for a small ESR width and a large ESR radius. Cross- and auto-correlation measures appear to be independent of lens image shear and convergence parameters, making it ideal to investigate ESR features. As with many microlensing experiments, the time-scale for variability, being of order decades to centuries, is impractically long. However, with a polarization filter oriented appropriately with respect to the path that the quasar takes across the caustic structure, the ESR diameter and radius can be estimated from the auto- and cross-correlation of polarized light curves on much shorter time-scales.