XMM-Newton detection of soft time lags in the TDE candidate AT 2018fyk

In this letter, we report a tentative detection of soft time lags (i.e. variability of softer photons lags behind the variability of harder photons) in one XMM-Newton observation of the tidal disruption event (TDE) candidate AT 2018fyk while the source was in the hard spectral state. The lags are detected at $6.51\times 10^{-5}~\rm Hz$. The amplitude of the lags with respect to 0.5 - 1 keV monotonically decreases with the photon energy, from $\sim 1200~\rm s$ at 0.3 - 0.5 keV to $\sim -4200~\rm s$ at 3 - 5 keV (in our convention, a positive lag means lagging behind the reference band). We find that the amplitude is proportional to the logarithm of the energy separation between the examined band and the reference band. The energy-dependent covariance spectrum indicates that the correlated variability is more likely to be associated with the non-thermal radiation. The soft lags are difficult to reconcile with the reverberation scenario that are used to explain the soft lags in active galactic nuclei. On the other hand, the observed soft lags are consistent with the picture that the soft X-rays are down-scattered hard X-rays by the outflow as predicted by 'unification' models of TDEs.