X-Ray Line Profile Variations during Quasar Microlensing

TL;DR

This paper models how gravitational microlensing causes specific, predictable changes in the X-ray iron line profiles of quasars, providing insights into the structure of the innermost accretion disk regions.

Contribution

It introduces a simulation of microlensing caustic crossings over a Kerr black hole's accretion disk, revealing how spectral features depend on disk geometry and caustic position.

Findings

Microlensing causes peaks and edges in line profiles.

Features depend on tangent points of $g$-factor contours.

Sequence of changes probes inner disk structure.

Abstract

Observations of several gravitationally microlensed quasars in X-rays revealed variations in the profile of the iron K$\alpha$ line in the course of microlensing events. We explore the effect by simulating a microlensing caustic crossing a spatially resolved model of emission from a thin accretion disk around a Kerr black hole. We demonstrate the sequence of spectral changes during the event, in particular the appearance of additional peaks and edges in the line profile due to microlensing. We trace the origin of these features to points on the disk, at which the total energy shift ($g$-factor) contours are tangent to the caustic. Contours tangent from the inner side of the caustic generate peaks, while those tangent from its outer side generate edges. We derive analytical shapes of the generated features and map the peak strength as a function of position of the tangent point on the disk. Since the features are determined by the positional geometry of the caustic relative to the $g$-factor contours, the same type of behavior can be expected in a much broader range of emission models. The sequence of line profile changes thus serves as a sensitive probe of the geometry and physics of the innermost region of the quasar accretion disk.