Observational Tests of Regular Black Holes with Scalar Hair and their Stability

TL;DR

This paper investigates the observable properties and stability of regular black holes with scalar hair, deriving constraints on the scalar charge from Solar System tests and black hole shadow observations.

Contribution

It introduces a detailed analysis of geodesics, light deflection, and shadow size in regular black holes with scalar hair, linking instability to observable features.

Findings

Stringent limits on scalar charge from Solar System tests.

Relations between photon sphere stability and black hole shadow size.

Increased scalar charge reduces photon orbit instability and enlarges the shadow.

Abstract

We study the geodesic structure and observable properties of asymptotically flat regular black holes sourced by a phantom scalar field characterized by a scalar charge $A$. This parameter removes the central singularity and continuously deforms the Schwarzschild geometry. The equations of motion for test particles and photons are derived, and the resulting null geodesics are analyzed, including the deflection of light, gravitational time delay, and redshift, in order to constrain $A$ using classical Solar System tests. These observations impose stringent limits on the scalar charge, confirming that $A$ must remain extremely small in the weak-field regime to ensure full consistency with general relativity. In the strong-field regime, we compute the Lyapunov exponent $\lambda$ associated with the photon sphere and establish its exact relations with the critical impact parameter $\mathcal{B}_u$ and the angular size of the shadow $\alpha_{\mathrm{sh}}$, given by $\mathcal{B}_u = 1/|\lambda|$ and $\alpha_{\mathrm{sh}} = 1/(r_{0}|\lambda|)$. These correspondences reveal that the dynamical instability of null circular orbits governs the optical appearance of the black hole. Our results show that increasing $A$ reduces the instability of photon trajectories and enlarges the angular size of the shadow, indicating that the regularization scale leaves a distinct observational imprint on the geometry of regular black holes. In addition, constraints derived from Event Horizon Telescope observations of M87* and Sgr A* further restrict the allowed range of the scalar charge, reinforcing the consistency of the model with current astrophysical observations.