Particle Dynamics and Thermal Properties in Kalb-Ramond ModMax Black Holes: Theoretical Predictions for Observational Tests of Exotic Physics

TL;DR

This paper provides a detailed theoretical analysis of Kalb-Ramond ModMax black holes, exploring their geodesic motion, thermodynamics, and observational signatures, highlighting how exotic parameters influence stability, phase transitions, and potential astrophysical observations.

Contribution

It introduces a comprehensive theoretical framework for Kalb-Ramond ModMax black holes, analyzing their unique geodesic, thermodynamic, and observational properties, including novel effects of LSB and nonlinearity parameters.

Findings

Shifted ISCO radii depending on parameters

Distinct thermodynamic phase structures and instabilities

Parameter-dependent gravitational lensing signatures

Abstract

We present a comprehensive theoretical study of geodesic motion and thermodynamic behavior in Kalb--Ramond (KR) black hole (BH) spacetimes sourced by ModMax electrodynamics. Both neutral and charged test particle dynamics are investigated, highlighting how the Lorentz symmetry breaking (LSB) parameter $\ell$, the ModMax nonlinearity parameter $\gamma$, and the discrete branch parameter $\zeta$ significantly modify orbital structures compared to classical Schwarzschild and Reissner--Nordstr\"{o}m (RN) solutions. Effective potential analysis reveals notable shifts in the innermost stable circular orbit (ISCO): ordinary branches allow stable orbits closer to the horizon, while phantom branches shift them outward by factors of 5--10. For charged particles, the combined influence of modified gravity and nonlinear electromagnetic fields may induce chaotic trajectories in certain regimes. On the thermodynamic side, we derive full expressions for Hawking temperature, entropy, and Helmholtz free energy. Ordinary branches exhibit divergent specific heat indicating second-order phase transitions, whereas phantom branches yield consistently negative specific heat, implying thermal instability. Phantom BHs are found to possess higher Hawking temperatures and show distinct thermodynamic phase structures, including Hawking--Page-type transitions. Observational features such as BH shadows and gravitational lensing are explored, revealing parameter-dependent changes in photon sphere radii and deflection angles. Notably, the deflection angle analysis via the Gauss--Bonnet theorem method (GBTm) shows opposite-sign electromagnetic corrections for phantom versus ordinary branches, suggesting potential observational discriminants.