Lensing by black holes within astrophysical environments

TL;DR

This paper investigates how realistic matter environments around black holes influence gravitational lensing effects using exact relativistic models, revealing significant impacts on lensing signals and gravitational wave echoes.

Contribution

It introduces a fully relativistic analysis of black hole lensing within a Hernquist matter halo, advancing beyond previous Newtonian or post-Newtonian approximations.

Findings

Large lensing 'bumps' are linked to halo parameters.

Environmental matter can significantly alter lensing inferences.

Gravitational wave echoes can be affected by the surrounding matter.

Abstract

Astrophysical black holes are likely to be surrounded by various forms of matter in the form of disks or halos. While a number of studies have examined the impact of an environment on the lensing of light or gravitational waves from cosmological sources, these have, thus far, been carried out in either a Newtonian or post-Newtonian framework where the environment is superimposed on the black-hole spacetime. By using an exact solution in general relativity describing a black hole embedded within a realistic halo of Hernquist matter distribution, we study deflection angles and image amplification in a fully relativistic setup. It is shown that large ``bumps'', that also arise at the Newtonian and post-Newtonian levels, track the transition scale set by the halo parameters that control the strong-lensing upturn and can significantly adjust the inferences made for either the source or lens in various contexts. As an application, we consider ``echoes'' of gravitational waves, sourced by astrophysical lenses rather than being intrinsic to the compact object that produces the signal.