Thin Accretion Disks around Rotating Charged Black Holes in an Effective Higher-Curvature Spacetime

TL;DR

This paper explores how higher-curvature modifications to rotating charged black hole spacetimes affect accretion disk properties, revealing potential observable deviations from classical Kerr black holes.

Contribution

It introduces a phenomenological deformation of the Kerr-Newman metric with an effective Gauss-Bonnet parameter and analyzes its impact on accretion disk observables.

Findings

Increasing deformation parameter $oldsymbol{eta}$ shifts the ISCO inward.

Higher $oldsymbol{eta}$ enhances disk radiation flux and temperature.

Charge $oldsymbol{Q}$ suppresses radiation flux and temperature.

Abstract

We investigate the structure and emission properties of a thin accretion disk around a rotating charged black hole described by an effective higher-curvature-inspired spacetime, constructed as a phenomenological deformation of the Kerr Newman geometry. In this framework, the deformation is introduced through a modification of the metric function $\Delta$ by an effective Gauss-Bonnet-like parameter $\alpha$, such that the spacetime reduces to the standard Kerr Newman solution in the limit $\alpha \to 0$. Adopting a kinematical approach, we use test-particle motion to derive the specific energy, specific angular momentum, and angular velocity of circular orbits, and analyze the effects of the parameters $\alpha$ and charge $Q$ on the innermost stable circular orbit (ISCO), radiative efficiency, radiation flux, temperature, and differential luminosity of the disk. We find that increasing $\alpha$ shifts the ISCO inward and enhances the disk's radiation flux and temperature, while the presence of charge suppresses these quantities due to electrostatic effects. Our results demonstrate that effective higher curvature deformations of rotating black hole spacetimes can lead to observable deviations from the Kerr case, highlighting accretion disks as sensitive probes of strong-gravity effects without relying on a specific underlying gravitational theory.