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When a star passes through the tidal disruption radius of a massive blackhole (BH), it can be torn apart by the tidal force of the BH, known as theTidal Disruption Event (TDE). Since the observed UV/optical luminositysignificantly exceeds the predictions of the compact disk model in classicalTDE theory, two competing models, stream collision and envelope reprocessing,have been proposed to address this discrepancy. To distinguish between thesemodels, we investigate the continuum reverberation behaviors for $\sim$30 TDEswith high-quality multi-band light curves. We found that over half of themexhibit a positive lag by a few days in UV/optical bands, indicating that theirinferred sizes are significantly larger than the envelope sizes inreprocessing. Moreover, X-ray emissions not only significantly delay relativeto the primary UV/optical peak but also lag behind the rebrightening bump by upto several tens of days, completely different from the X-ray illuminationreprocessing. Additionally, the anti-correlated UV-optical continuum inASASSN-15lh further disfavors the reprocessing scenario. In contrast, the modelof stream collisions, combined with delayed accretion disk formation, canprovide a unified explanation for the diverse TDE observations, e.g., theoptical/X-ray population, the frequently observed rebrightening bump. Thismodel describes a unification scheme wherein the UV/optical emission originatesfrom stream collisions during the early-stage of TDE evolution and graduallytransitions to being dominated by accretion disk with detectable X-ray emissionin the late stage. After transitioning to a quiescent state, recurrent flaresmay be observed in some cases, possibly related to repeating partial TDEs.

Reverberation evidence for Stream Collision and Delayed Disk Formation in Tidal Disruption Events