Shadows of Exact Binary Black Holes

TL;DR

This paper demonstrates that the main features of black hole shadows in binary systems can be understood through a simplified quasi-static model, and explores how spin and orbital motion influence these shadows.

Contribution

It introduces a simple quasi-static approach to model binary black hole shadows, capturing key features without complex numerical relativity simulations.

Findings

Shadows are well approximated by static double-Schwarzschild solutions.

Intrinsic spin affects the shape and size of the shadows.

Orbital angular momentum influences the shadow morphology.

Abstract

Black hole (BH) shadows in dynamical binary BHs (BBHs) have been produced via ray-tracing techniques on top of expensive fully non-linear numerical relativity simulations. We show that the main features of these shadows are captured by a simple quasi-static resolution of the photon orbits on top of the static double-Schwarzschild family of solutions. Whilst the latter contains a conical singularity between the line separating the two BHs, this produces no major observable effect on the shadows, by virtue of the underlying cylindrical symmetry of the problem. This symmetry is also present in the stationary BBH solution comprising two Kerr BHs separated by a massless strut. We produce images of the shadows of the exact stationary co-rotating (even) and counter-rotating (odd) stationary BBH configurations. This allow us to assess the impact on the binary shadows of the intrinsic spin of the BHs, contrasting it with the effect of the orbital angular momentum.