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We carried out the first multi-wavelength (optical/UV and X-ray) photometricreverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We findthat its X-ray variations are correlated with and lag the optical/UVfluctuations by 32$\pm$4 days. Based on the direction and the magnitude of theX-ray time lag, we rule out X-ray reprocessing and direct emission from astandard circular thin disk as the dominant source of its optical/UV emission.The lag magnitude also rules out an AGN disk-driven instability as the originof ASASSN-14li and thus strongly supports the tidal disruption picture for thisevent and similar objects. We suggest that the majority of the optical/UVemission likely originates from debris stream self-interactions. Perturbationsat the self-interaction sites produce optical/UV variability and travel down tothe black hole where they modulate the X-rays. The time lag between theoptical/UV and the X-rays variations thus correspond to the time taken by thesefluctuations to travel from the self-interaction site to close to the blackhole. We further discuss these time lags within the context of the threevariants of the self-interaction model. High-cadence monitoring observations offuture TDFs will be sensitive enough to detect these echoes and would allow usto establish the origin of optical/UV emission in TDFs in general.

Optical/UV-to-X-Ray Echoes from the Tidal Disruption Flare ASASSN-14li