Black Holes in Proca-Gauss-Bonnet Gravity with Primary Hair: Particle Motion, Shadows, and Grey-Body Factors

Abstract: We investigate classical and semiclassical signatures of black holes in a recently proposed Proca-Gauss-Bonnet gravity model that admits asymptotically flat solutions with primary hair. Two distinct classes of spherically symmetric metrics arise from different relations between the coupling constants of scalar-tensor and vector-tensor Gauss-Bonnet interactions. For each geometry, we examine the range of parameters permitting horizon formation and analyze the motion of test particles and light rays. We compute characteristic observables including the shadow radius, Lyapunov exponent, innermost stable circular orbit (ISCO) frequency, and binding energy. Additionally, we study scalar and Dirac field perturbations, derive the corresponding effective potentials, and calculate the grey-body factors (GBFs) using both the sixth-order Wentzel-Kramers-Brillouin (WKB) method and their correspondence with quasinormal modes (QNMs). Our results show that the QNM-based approximation of GBFs is accurate for sufficiently large multipole numbers and that deviations from Schwarzschild geometry become pronounced for large values of the Proca hair and Gauss-Bonnet couplings.