标题： XMM-Newton对暂现源AT 2018fyk中软时间滞后现象的探测 显示英文标题

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

摘要： 在本文中，我们报告在潮汐破坏事件（TDE）候选体AT 2018fyk的一个$XMM-Newton$观测中发现了软时间滞后（即较软光子的变异性滞后于较硬光子的变异性），此时源处于硬谱态。滞后现象在$6.51\times10^{-5}~\rm Hz$被检测到。相对于0.5$-$1 keV，滞后幅度随着光子能量单调减小，从0.3$-$0.5 keV处的$\sim 1200~\rm s$到3$-$5 keV处的$\sim -4200~\rm s$（在我们的惯例中，正的滞后意味着滞后于参考波段）。我们发现，幅度与所研究波段与参考波段之间的能量间隔的对数成正比。能量依赖的协方差谱表明，相关变异性更可能与非热辐射有关。软滞后难以与用于解释活动星系核中软滞后的回响情景相协调。另一方面，观测到的软滞后与软X射线由外流散射硬X射线的图像一致，这符合TDE“统一”模型的预测。 显示英文摘要

摘要： 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 $6.51\times10^{-5}~\rm Hz$. 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.