Two families of astrophysical diverging lens models

TL;DR

This paper investigates two types of astrophysical diverging plasma lens models, revealing their distinct image formation behaviors and potential to explain frequency-dependent phenomena like extreme scattering events.

Contribution

It introduces and analyzes power-law and exponential plasma lens models, highlighting their unique features and similarities to Gaussian lenses, with implications for multi-wavelength astrophysical observations.

Findings

Models show distinct image formation and magnification behaviors.

Finite core in power-law lenses can produce caustics similar to Gaussian lenses.

Wavelength-dependent deflections enable constraining electron density distributions.

Abstract

In the standard gravitational lensing scenario, rays from a background source are bent in the direction of a foreground lensing mass distribution. Diverging lens behaviour produces deflections in the opposite sense to gravitational lensing, and is also of astrophysical interest. In fact, diverging lensing due to compact distributions of plasma has been proposed as an explanation for the extreme scattering events (ESEs) that produce frequency-dependent dimming of extra-galactic radio sources, and may also be related to the refractive radio-wave phenomena observed to affect the flux density of pulsars. In this work we study the behaviour of two families of astrophysical diverging lenses in the geometric optics limit, the power-law and the exponential plasma lenses. Generally, the members of these model families show distinct behaviour in terms of image formation and magnification, however the inclusion of a finite core for certain power-law lenses can produce a caustic and critical curve morphology that is similar to the well-studied Gaussian plasma lens. Both model families can produce dual radial critical curves, a novel distinction from the tangential distortion usually produced by gravitational (converging) lenses. The deflection angle and magnification of a plasma lens varies with the observational frequency, producing wavelength-dependent magnifications that alter the amplitudes and the shape of the light curves. Thus, multi-wavelength observations can be used to physically constrain the distribution of the electron density in such lenses.