Do they repeat? Monitoring 36 non-repeating FRBs with FAST

The origin of fast radio bursts (FRBs), highly energetic millisecond-duration radio pulses originating from beyond our Galaxy, remains unknown. Observationally, FRBs are classified as non-repeating or repeating; however, this classification is complicated by limited observing time and sensitivity constraints, which may result in some repeating FRBs being misidentified as non-repeating. To address this issue, we adopt both empirical and machine-learning techniques from previous studies to identify candidates that may have been misclassified. We conducted follow-up observations of 36 such candidates, each observed for 10 min using the Five-hundred-meter Aperture Spherical Telescope (FAST). No radio bursts exceeding a signal-to-noise ratio of 7 were detected, with a typical 7σ fluence limit of ~0.013 Jy ms. We constrain the repetition rates of these sources using two statistical models of FRB occurrence. Combining our FAST non-detections with prior observations, we derive upper limits on the repetition rates of ~ 10-2.6–10-0.22 h⁻¹ under a Poisson process, and ~ 10-2.3–10-0.25 h⁻¹ under a Weibull process. This work presents one of the most stringent upper limits on FRB repetition rates to date, based on a sample size five times larger than those used in previous studies.