Long-term Evolution of the Short-term X-ray Variability of the Jetted
  TDE Swift J1644+57
  The short-term X-ray variability of tidal disruption events (TDEs) and its
similarities with active galactic nuclei (AGN) are poorly understood. In this
work, we show the diversity of TDE's short-term X-ray variability, and take
Swift J1644+57 as an example to study the evolution of various properties
related to the short-term X-ray variability, such as the X-ray flux
distribution, power spectral density (PSD), rms variability, time lag and
coherence spectra. We find that the flux distribution of Swift J1644+57 has a
lognormal form in the normal state, but deviates from it significantly in the
dipping state, thereby implying different physical mechanisms in the two
states. We also find that during the first two XMM-Newton observations in the
dipping state, Swift J1644+57 exhibited different variability patterns, which
are characterized by steeper PSDs and larger rms than the normal state. A
significant soft X-ray lag is detected in these two observations, which is ~50
s between 0.3-1 keV and 2-10 keV with a high coherence. Using the 2-10 keV rms
of 0.10-0.50, the black hole mass of Swift J1644+57 is estimated to be
$(0.6-7.9)\times10^{6}M_{\odot}$, but the variation of rms as the TDE evolves
introduces a large uncertainty. Finally, we discuss the value of conducting
similar studies on other TDEs, especially in the coming era of time-domain
astronomy when a lot more TDEs will be discovered in X-rays promptly. This also
heralds a significant increase in the demand for deep follow-up observations of
X-ray selected TDEs with X-ray telescopes of large effective areas and long
orbital periods.
