Lensing by a singular isothermal sphere and a black hole

TL;DR

This paper models gravitational lensing by a supermassive black hole offset from a galaxy's center, analyzing image configurations and magnifications, with implications for future high-resolution astronomical observations.

Contribution

It introduces a model combining a singular isothermal sphere galaxy with an off-centre black hole as a point lens, analyzing critical curves, caustics, and image multiplicities.

Findings

Number of images varies from two to five depending on parameters.

Image magnification perturbation scales linearly or quadratically with black hole mass.

Highly de-magnified images near the centre are challenging to observe.

Abstract

Most galaxies host central supermassive black holes. As two galaxies merge, the black holes also merge. The final single black hole may suffer a kick due to asymmetric gravitational radiation and may not be at the centre of the galaxy; off-centre black holes may also be produced by other means such as sustained acceleration due to asymmetric jet power. We model the main galaxy as a singular isothermal sphere and the black hole as an off-centre point lens, and study the critical curves and caustics using complex notation. We identify the critical parameters that govern the transitions in the topology of critical curves, caustics and pseudo-caustics, and find the number of images can be two, three, four and five. We show examples of image configurations, including cases where three highly de-magnified images are found close to the centre. The perturbation on the image magnification due to the black hole scales linearly with its mass in the off-centre case, and quadratically when the black hole is at the centre. Such images are difficult to observe unless high-contrast and high-resolution imaging facilities (e.g., the Square Kilometer Array in the radio) become available.