# What shadow does a binary black hole cast?

### A new and simple way to calculate shadows of dynamical binary black holes without expensive numerical-relativity computations

August 31, 2018

*Left:*Shadows and lensing of a fully dynamical binary of equal-mass BHs with no spin (adapted from [19]).

*Right:*Quasi-static BH binary with z

_{o}= 1.5M and ω = 0.06M

^{-1}.

## Paper 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.