Probing the Scalar Hair of Rotating Horndeski Black Holes through Thick Disk Images

TL;DR

This paper models thick accretion disks around rotating Horndeski black holes to identify observable effects of scalar hair on images, highlighting the photon ring diameter as a key probe.

Contribution

It introduces an analytical model of a magnetized thick disk around hairy black holes and identifies the photon ring diameter as a promising observable for detecting scalar hair.

Findings

Scalar hair modestly affects inflow and magnetic fields.

Scalar hair increases gravitational redshift, reducing flux and brightness.

Photon ring diameter strongly depends on hair parameter, less on accretion details.

Abstract

Horizon-scale images of black holes provide a potential probe of fundamental physics, including tests of gravity and black hole hair. To assess the impact of scalar hair on accretion-flow imaging self-consistently, we construct an analytical model of a geometrically thick, magnetized disk around a rotating hairy black hole in Horndeski theory and analyze its 230 GHz image morphology. We find that scalar hair modestly alters the inflow and magnetic-field structure but strengthens gravitational redshift, markedly reducing the total flux and lensed ring brightness through relativistic transfer and spectral-shift effects. Moreover, we highlight a previously unexplored channel: the maximum interferometric diameter of the first photon ring responds strongly to the hair parameter but shows little dependence on accretion-flow details, making it a promising observable for constraining black-hole hair with future space-based interferometry.