We carried out the first multi-wavelength (optical/UV and X-ray) photometric reverberation mapping of a tidal disruption flare (TDF) ASASSN-14li. We find that its X-ray variations are correlated with and lag the optical/UV fluctuations by 32 ± 4 days. Based on the direction and the magnitude of the X-ray time lag, we rule out X-ray reprocessing and direct emission from a standard 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 origin of ASASSN-14li and thus strongly supports the tidal disruption picture for this event and similar objects. We suggest that the majority of the optical/UV emission likely originates from debris stream self-interactions. Perturbations at the self-interaction sites produce optical/UV variability and travel down to the black hole where they modulate the X-rays. The time lag between the optical/UV and the X-rays variations thus correspond to the time taken by these fluctuations to travel from the self-interaction site to close to the black hole. We further discuss these time lags within the context of the three variants of the self-interaction model. High-cadence monitoring observations of future TDFs will be sensitive enough to detect these echoes and would allow us to establish the origin of optical/UV emission in TDFs in general.
Bibliographical noteFunding Information:
This work is based on observations made with Swift, a mission that was managed and controlled by NASAs Goddard Space Flight Center (GSFC) in Greenbelt, MD, USA. All the data used in the present article are publicly available through NASAs HEASARC archive. J.G. would like to thank ChelseaMacLeod, Anna Pancoast, and Yanfei Jiang for valuable discussions. Finally, D.R.P. would like to thank Poshak Gandhi, Simon White, and Margaret Trippe for valuable comments and suggestions.
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- accretion, accretion disks
- black hole physics