# 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

Black holes have shadows: light from the distance passing close to a black hole's event horizons get swallowed and therefore a shadow region is cast behind the black hole. The region is larger than the geometric projection of the event horizon and depends on the mass and the spin of the black hole. In turn, observations of these shadows can be used to infer properties of the black hole. Here, researchers show how to simply obtain the shadows cast by black holes orbiting around their common center of mass. They combine a simple quasi-static tracing of photon orbits with the static double-Schwarzschild family of solutions. They also calculate shadows of exact stationary rotating Kerr black holes.

## 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.