Finding diamonds in the rough: Targeted Sub-threshold Search for Strongly-lensed Gravitational-wave Events

Strong gravitational lensing of gravitational waves can produce duplicate signals separated in time with different amplitudes. We consider the case in which strong lensing produces identifiable gravitational-wave events and weaker sub-threshold signals buried in the noise background. We present the \gstlal-based TargetEd Sub-threshold Lensing seArch (TESLA) search method for the sub-threshold signals using reduced template banks targeting specific confirmed gravitational-wave events. We perform a mock data challenge to assess the performance of the proposed search method and show that it can effectively uprank potential sub-threshold lensed counterparts to the target gravitational-wave event. We also compare its performance to other alternative solutions to the posed problem and demonstrate that our proposed method outperforms the other solutions.