Analytical and numerical study of accretion processes around charged spherically symmetric black holes in scalar-tensor Gauss-Bonnet gravity

TL;DR

This study combines analytical and numerical methods to explore how scalar-tensor Gauss-Bonnet gravity influences accretion processes around charged black holes, revealing modifications in orbital dynamics and accretion flow morphology.

Contribution

It provides the first detailed analysis of accretion phenomena around charged black holes within scalar-tensor Gauss-Bonnet gravity, highlighting the impact of modified gravity parameters on accretion characteristics.

Findings

Increased Gauss-Bonnet coupling reduces gravitational focusing.

Negative cosmological constant widens the shock cone.

Accretion rate and density decrease with stronger scalar-tensor effects.

Abstract

We investigate the physical phenomena occurring around a spherically symmetric, non-rotating charged black hole (BH) to explore the effects of scalar-tensor Gauss-Bonnet gravity on circular motion, accretion disk properties, and Bondi-Hoyle-Lyttleton (BHL) accretion flow. By analytically and numerically examining the influence of the Gauss-Bonnet coupling constant $c_1$ and the cosmological parameter $\Lambda$, we reveal how these modified gravity parameters alter the underlying physical processes. Using geodesic analysis, we compute the specific energy, angular momentum, innermost stable circular orbit (ISCO) radius, and radiation flux of test particles, providing insight into how the modified gravity framework affects orbital stability and the organization of the accretion flow. Subsequently, through numerical solutions of the general relativistic hydrodynamic (GRHD) equations, we describe the morphology of the shock cone formed via the BHL accretion mechanism around the BH. The numerical results demonstrate that increasing the values of $c_1$ and negative $\Lambda$ reduce gravitational focusing. Consequently, depending on the parameter choices, the opening angle of the shock cone either widens or narrows compared to the Schwarzschild case. However, because of weakened gravitational focusing, both the amount of accreted matter and the density of material trapped inside the cone decrease significantly. These results indicate that scalar-tensor Gauss-Bonnet corrections act as an effective gravitational damping term, transferring turbulence and transforming shock-dominated accretion into more stable configurations. The consistency between theoretical and numerical results suggests that the observable properties of accretion disks and quasi-periodic oscillations (QPOs) can serve as probes to constrain the parameters of scalar-tensor Gauss-Bonnet gravity in strong-field regimes.