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It has been argued that ultracompact objects, which possess light rings butno horizons, may be unstable against gravitational perturbations. To test thisconjecture, we revisit the quasi-black hole solutions, a family of horizonlessspacetimes whose limit is the extremal Reissner-Nordstr\"om black hole. We finda critical parameter at which the light rings just appear. We then calculatethe quasinormal modes of the quasi-black holes. Both the WKB result and thenumerical result show that long-live modes survive for the range where lightrings exist, indicating that horizonless spacetimes with light rings areunstable. Our work provides a strong and explicit example that light ringscould be direct observational evidence for black holes.

Light rings and long-lived modes in quasi-black hole spacetimes