Images of the Thin Accretion Disk Around Kerr Black Holes coupled to time periodic scalar fields

TL;DR

This paper explores how scalar hair around Kerr black holes alters the structure and appearance of accretion disks, revealing potential observational signatures for testing alternative gravity theories.

Contribution

It introduces a detailed analysis of accretion disk properties around Kerr black holes with scalar hair, highlighting observable deviations from standard Kerr predictions.

Findings

Scalar hair significantly modifies disk luminosity and morphology.

Presence of additional light rings and inner emitting zones.

Enhanced redshift patterns and distinctive frequency shifts in the emission.

Abstract

We investigate the orbital structure and observable appearance of rotating Kerr black holes endowed with synchronized scalar hair described by two time-periodic scalar fields with a flat target-space geometry. The presence of scalar hair enriches the geodesic structure of the spacetime relative to the Kerr case and significantly modifies the emission properties of geometrically thin Novikov-Thorne accretion disks. Combining an analysis of timelike circular orbits with backward ray tracing, we show that the normalized scalar charge governs the morphology and luminosity of both prograde and counter-rotating disks. In the strongly scalarized regime, additional light rings and modified circular-orbit regions produce multiple inner emitting zones and strongly enhanced redshift patterns that depart markedly from the Kerr prediction. The most pronounced deviations occur in the counter-rotating sector, where scalar hair generates inner retrograde radiative rings with substantially enhanced luminosity and distinctive frequency-shift signatures. Even when the spacetime approaches the Kerr geometry at weaker scalarization, the retrograde disk remains highly sensitive to the presence of scalar hair. Our results demonstrate that geometrically thin accretion disks can provide robust observational diagnostics of synchronized scalar hair and may offer a promising avenue for testing tensor-multi-scalar gravity with future horizon-scale black-hole imaging observations.