Microlensing as a possible probe of event-horizon structure in quasars

TL;DR

This paper explores how microlensing-induced brightness variations in lensed quasars can reveal the shape of the quasar's central engine, using a toy model of crescent-shaped sources crossing caustics.

Contribution

It introduces a toy model demonstrating how crescent-shaped quasar cores produce distinct microlensing light curves, enabling shape inference from observational data.

Findings

Crescent-shaped sources produce unique microlensing signatures.

Good monitoring data can recover crescent parameters.

The silhouette of a black hole can be inferred from light curves.

Abstract

In quasars which are lensed by galaxies, the point-like images sometimes show sharp and uncorrelated brightness variations (microlensing). These brightness changes are associated with the innermost region of the quasar passing through a complicated pattern of caustics produced by the stars in the lensing galaxy. In this paper, we study whether the universal properties of optical caustics could enable extraction of shape information about the central engine of quasars. We present a toy model with a crescent-shaped source crossing a fold caustic. The silhouette of a black hole over an accretion disk tends to produce roughly crescent sources. When a crescent-shaped source crosses a fold caustic, the resulting light curve is noticeably different from the case of a circular luminosity profile or Gaussian source. With good enough monitoring data, the crescent parameters, apart from one degeneracy, can be recovered.