Essentials of strong gravitational lensing

TL;DR

This paper introduces the fundamental concepts and mathematical tools of strong gravitational lensing, emphasizing physical insights and discussing both basic and advanced ideas, to aid understanding of this astrophysical phenomenon.

Contribution

It provides a comprehensive, accessible overview of strong gravitational lensing, including key constructs like the Fermat potential and methods to go beyond common simplifying assumptions.

Findings

Explains how wavefront arguments extend lensing theory to strong fields.

Describes how image configurations are formed from saddle points.

Discusses why strong lensing is most common at cosmological distances.

Abstract

Of order one in 10^3 quasars and high-redshift galaxies appears in the sky as multiple images as a result of gravitational lensing by unrelated galaxies and clusters that happen to be in the foreground. While the basic phenomenon is a straightforward consequence of general relativity, there are many non-obvious consequences that make multiple-image lensing systems (aka strong gravitational lenses) remarkable astrophysical probes in several different ways. This article is an introduction to the essential concepts and terminology in this area, emphasizing physical insight. The key construct is the Fermat potential or arrival-time surface: from it the standard lens equation, and the notions of image parities, magnification, critical curves, caustics, and degeneracies all follow. The advantages and limitations of the usual simplifying assumptions (geometrical optics, small angles, weak fields, thin lenses) are noted, and to the extent possible briefly, it is explained how to go beyond these. Some less well-known ideas are discussed at length: arguments using wavefronts show that much of the theory carries over unchanged to the regime of strong gravitational fields; saddle-point contours explain how even the most complicated image configurations are made up of just two ingredients. Orders of magnitude, and the question of why strong lensing is most common for objects at cosmological distance, are also discussed. The challenges of lens modeling, and diverse strategies developed to overcome them, are discussed in general terms, without many technical details.