It has been argued that ultracompact objects, which possess light rings but no horizons, may be unstable against gravitational perturbations. To test this conjecture, we revisit the quasiblack hole solutions, a family of horizonless spacetimes whose limit is the extremal Reissner-Nordström black hole. We find a critical parameter at which the light rings just appear. We then calculate the quasinormal modes of the quasiblack holes. Both the WKB result and the numerical result show that long-lived modes survive for the range where light rings exist, indicating that horizonless spacetimes with light rings are unstable. Our work provides a strong and explicit example that light rings could be direct observational evidence for black holes.

• Received 23 August 2021
• Revised 26 October 2021
• Accepted 6 January 2022

DOI:https://doi.org/10.1103/PhysRevD.105.024049