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

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 Eq.~(9) of Ref.~\cite{BakopoulosEtAl2024}. Working in a weak-hair regime for the coupling combination $β\equivηq<sup>4$, we derive closed first-order formulas for the photon-sphere radius, orbital frequency $Ω<em>{\text{ph}}$, and Lyapunov exponent $λ</em>{\text{ph}}$. 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 $Γ_\ell(ω)$. 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 ($\ell=3,4$) and several core-size ratios. The resulting construction provides a concise one-parameter connection from the metric function to ringdown, lensing, and scattering observables.