Rate of repeating tidal disruption events with a 5–19 years interval

Context. Statistics on tidal disruption events (TDEs) may be contaminated by repeating TDEs (rTDEs), which have been widely discovered in recent years. However, no statistical study has yet examined rTDEs with time intervals longer than 5 years. In addition, the origin of rTDEs remains unclear. Aims. We aim to search for rTDEs with time intervals longer than 5 years in a well-defined TDE sample and to estimate the rTDE rate and fraction in the sample. Methods. We used a sample of 16 TDEs at z < 0.05 from the Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS) to search for flares 5–19 years before the ZTF TDEs using the Catalina Real-time Sky Survey (CRTS) light curves. We analyzed archival multi-band data to distinguish between TDEs and supernovae (SNe) and estimated the expected number of SNe that CRTS could detect in the sample. Results. We identify two rTDE candidates, AT 2019azh and AT 2024pvu, with time intervals of 13.2 and 17.1 years, respectively. The peak luminosities of the CRTS flares are close to those of ZTF flares. For the CRTS flare of AT 2024pvu, we used UV observations from the Galaxy Evolution Explorer (GALEX) near the peak to measure a blackbody temperature of ∼19 500 K, consistent with TDEs and higher than that of SNe. Moreover, we estimate the expected number of SNe in the sample to be ≲0.08, and hence the probability that both CRTS flares are SNe is only 0.3%. Therefore, we rule out the possibility that both CRTS flares are SNe and conclude that both are likely TDEs. Using the two rTDEs, we infer that the TDE rate is two to three orders of magnitude higher than the average over 5–19 years prior to TDE detection. Two rTDEs with intervals of ∼2 years in the sample, together with possible rTDEs missed by CRTS, suggest that rTDEs with intervals of < 20 years may account for 25%–60% of the TDE sample. We interpret rTDEs as repeating partial TDEs. If so, the high fraction of rTDEs suggests that the observed optical TDE rate is overestimated. However, the possibility of independent TDEs cannot be ruled out and requires future observational tests.