Effects of supermassive binary black holes on gravitational lenses

TL;DR

This paper investigates how supermassive binary black holes influence gravitational lensing, revealing they can produce additional observable images and affect core image properties, which is crucial for future high-resolution lens observations.

Contribution

It provides a detailed analysis of the impact of SMBBHs on lensing features, including critical curves, caustics, and image magnifications, highlighting potential observational signatures.

Findings

Binary black holes can create at least one additional image in lensing systems.

The probability of detecting high-magnification images due to SMBBHs is significant in multiply-imaged systems.

SMBBHs suppress the formation of faint core images, affecting constraints on galaxy core sizes.

Abstract

Recent observations indicate that many if not all galaxies host massive central black holes (BHs). In this paper we explore the influence of supermassive binary black holes (SMBBHs) on their actions as gravitational lenses. When lenses are modelled as singular isothermal ellipsoids, binary black holes change the critical curves and caustics differently as a function of distance. Each black hole can in principle create at least one additional image, which, if observed, provides evidence of black holes. By studying how SMBBHs affect the cumulative distribution of magnification for images created by black holes, we find that the cross section for at least one such additional image to have a magnification larger than $10^{-5}$ is comparable to the cross section for producing multiple-images in singular isothermal lenses. Such additional images may be detectable with high-resolution and large dynamic range maps of multiply-imaged systems from future facilities, such as the SKA. The probability of detecting at least one image (two images) with magnification above $10^{-3}$ is $\sim 0.2 \fBH$ ($\sim 0.05 \fBH$) in a multiply-imaged lens system, where $\fBH$ is the fraction of galaxies housing binary black holes. We also study the effects of SMBBHs on the core images when galaxies have shallower central density profiles (modelled as non-singular isothermal ellipsoids). We find that the cross section of the usually faint core images is further suppressed by SMBBHs. Thus their presence should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses.