A generalization of photon sphere based on escape/capture coneMasaya Amo, Keisuke Izumi, Hirotaka Yoshino, Yoshimune Tomikawa and Tetsuya ShiromizuEur. Phys. J. C, 84 6 (2024) 638DOI: https://doi.org/10.1140/epjc/s10052-024-12954-9