A First-Order Eikonal Framework for Quasinormal Modes, Shadows, Strong Lensing, and Grey-Body Factors in a Scalarized Black-Hole Metric

TL;DR

This paper develops an analytic framework connecting black-hole metrics to observable phenomena like quasinormal modes, shadows, and lensing, using a first-order eikonal approximation applicable to scalarized black holes.

Contribution

It introduces a closed-form, first-order eikonal approach linking black-hole geometry to multiple observables, extending the standard shadow–ringdown correspondence.

Findings

Derived formulas for photon-sphere radius, orbital frequency, and Lyapunov exponent.

Established a universal relation between quasinormal modes and shadow/deflection observables.

Analyzed grey-body factors and damping ratios across different core sizes.

Abstract

We construct an analytic geodesic-optics description of quasinormal ringing, black-hole shadows, strong lensing, and grey-body factors for the static spherical metric introduced in [Bakopoulos, et. al. arXiv:2310.11919]. Working in a weak-hair regime, we derive closed first-order formulas for the photon-sphere radius, orbital frequency, and Lyapunov exponent. These invariants are then employed within the Schutz--Will WKB approach to obtain eikonal quasinormal frequencies, mapped to shadow and strong-deflection observables through exact identities for static spherical geometries, and used to build a closed analytic form for the transmission probability. At leading eikonal order, these relations are controlled by null geodesics and are therefore spin-universal for test scalar/electromagnetic/gravitational sectors, up to subleading corrections. Besides the standard ringdown--shadow correspondence, we present three additional results: (i) an explicit quality-factor correction, (ii) limiting core-size expansions that show when damping ratios are nearly insensitive to the scalarized core, and (iii) a comparative study of grey-body factors for moderate multipoles and several core-size ratios. The resulting construction provides a concise one-parameter connection from the metric function to ringdown, lensing, and scattering observables.