Shadow of a disformal Kerr black hole in quadratic DHOST theories

TL;DR

This paper investigates how a disformal Kerr black hole's shadow in quadratic DHOST theories varies with deformation and spin, revealing distinctive features like 'pedicel'-like structures and fractal patterns that could inform tests of alternative gravity theories.

Contribution

It introduces the study of black hole shadows in non-stealth disformal Kerr solutions within quadratic DHOST theories, highlighting new shadow features linked to scalar field effects.

Findings

Shadow size increases with deformation parameter.

Shadow shape depends on black hole spin and deformation sign.

Distinctive features like 'pedicel' and fractal structures appear in shadows.

Abstract

We have studied the shadow of a disformal Kerr black hole with an extra deformation parameter, which belongs to non-stealth rotating solutions in quadratic Degenerate Higher Order Scalar Tensor (DHOST) theory. Our result show that the size of the shadow increases with the deformation parameter for the black hole with arbitrary spin parameter. However, the effect of the deformation parameter on the shadow shape depends heavily on the spin parameter of black hole and the sign of the deformation parameter. The change of the shadow shape becomes more distinct for the black hole with the more quickly rotation and the more negative deformation parameter. Especially, for the near-extreme black hole with negative deformation parameter, there exist a "pedicel"-like structure appeared in the shadow, which increases with the absolute value of deformation parameter. The eyebrow-like shadow and the self-similar fractal structures also appear in the shadow for the disformal Kerr black hole in DHOST theory. These features in the black hole shadow originating from the scalar field could help us to understand the non-stealth disformal Kerr black hole and quadratic DHOST theory.