Shadows of Kerr black holes with scalar hair

TL;DR

This paper investigates the shadows of Kerr black holes with scalar hair (KBHsSH), revealing new shadow shapes and features that differ from standard Kerr black holes, with implications for observational astronomy.

Contribution

It introduces a detailed analysis of KBHsSH shadows, highlighting their unique shapes and multiple shadow possibilities, expanding the understanding of black hole imaging.

Findings

KBHsSH shadows can be drastically different from Kerr black holes.

Ultra-compact boson stars produce infinite images with self-similar structures.

KBHsSH provide new templates for black hole shadow observations.

Abstract

Using backwards ray tracing, we study the shadows of Kerr black holes with scalar hair (KBHsSH). KBHsSH interpolate continuously between Kerr BHs and boson stars (BSs), so we start by investigating the lensing of light due to BSs. Moving from the weak to the strong gravity region, BSs - which by themselves have no shadows - are classified, according to the lensing produced, as: $(i)$ non-compact, which yield no multiple images; $(ii)$ compact, which produce an increasing number of Einstein rings and multiple images of the whole celestial sphere; $(iii)$ ultra-compact, which possess light rings, yielding an infinite number of images with (we conjecture) a self-similar structure. The shadows of KBHsSH, for Kerr-like horizons and non-compact BS-like hair, are analogous to, but distinguishable from, those of comparable Kerr BHs. But for non-Kerr-like horizons and ultra-compact BS-like hair, the shadows of KBHsSH are drastically different: novel shapes arise, sizes are considerably smaller and multiple shadows of a single BH become possible. Thus, KBHsSH provide quantitatively and qualitatively new templates for ongoing (and future) very large baseline interferometry (VLBI) observations of BH shadows, such as those of the Event Horizon Telescope.