Shadows of Kerr black holes with and without scalar hair

TL;DR

This paper explores the shadows cast by Kerr black holes with scalar hair, revealing smaller and potentially exotic shadow shapes, which could aid future observational tests like those by the Event Horizon Telescope.

Contribution

It introduces a backward ray-tracing algorithm for shadow computation and derives an analytical Kerr shadow solution for a ZAMO observer at any position.

Findings

Kerr black holes with scalar hair have smaller shadows than standard Kerr black holes.

Exotic shadow shapes can appear under certain conditions.

An analytical Kerr shadow solution is provided for a ZAMO observer.

Abstract

For an observer, the Black Hole (BH) shadow is the BH's apparent image in the sky due to the gravitational lensing of nearby radiation, emitted by an external source. A recent class of solutions dubbed Kerr BHs with scalar hair possess smaller shadows than the corresponding Kerr BHs and, under some conditions, novel exotic shadow shapes can arise. Thus, these hairy BHs could potentially provide new shadow templates for future experiments such as the Event Horizon Telescope. In order to obtain the shadows, the backward ray-tracing algorithm is briefly introduced, followed by numerical examples of shadows of Kerr BHs with scalar hair contrasting with the Kerr analogues. Additionally, an analytical solution for the Kerr shadow is derived in closed form for a ZAMO observer at an arbitrary position.