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

## TL;DR

This study presents the first multi-wavelength reverberation mapping of a tidal disruption flare, revealing that optical/UV emission likely originates from debris stream interactions, with X-ray variations lagging behind by about 32 days.

## Contribution

It provides new evidence supporting debris stream self-interactions as the source of optical/UV emission in TDFs, challenging previous reprocessing or disk instability models.

## Key findings

- X-ray variations lag optical/UV by 32 days
- Rules out standard disk reprocessing as dominant emission source
- Supports debris stream self-interactions as origin of optical/UV emission

## Abstract

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$\pm$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.