Long-term X-ray variability of Swift J1644+57
  We studied the X-ray timing and spectral variability of the X-ray source Sw
J1644+57, a candidate for a tidal disruption event. We have separated the
long-term trend (an initial decline followed by a plateau) from the short-term
dips in the Swift light-curve. Power spectra and Lomb-Scargle periodograms hint
at possible periodic modulation. By using structure function analysis, we have
shown that the dips were not random but occurred preferentially at time
intervals ~ [2.3, 4.5, 9] x 10^5 s and their higher-order multiples. After the
plateau epoch, dipping resumed at ~ [0.7, 1.4] x 10^6 s and their multiples. We
have also found that the X-ray spectrum became much softer during each of the
early dip, while the spectrum outside the dips became mildly harder in its
long-term evolution. We propose that the jet in the system undergoes precession
and nutation, which causes the collimated core of the jet briefly to go out of
our line of sight. The combined effects of precession and nutation provide a
natural explanation for the peculiar patterns of the dips. We interpret the
slow hardening of the baseline flux as a transition from an extended, optically
thin emission region to a compact, more opaque emission core at the base of the
jet.
