AT2019 avd: A tidal disruption event with a two-phase evolution
  Tidal disruption events (TDEs) can uncover the quiescent supermassive black
holes (SMBHs) at the center of galaxies. After the disruption of a star by a
SMBH, the highly elliptical orbit of the debris stream will be gradually
circularized due to the self-crossing, and then the circularized debris will
form an accretion disk. The recent TDE candidate AT 2019avd has double peaks in
its optical light curve, and the X-ray emerges near the second peak. The
durations of the peaks are ~400 and 600 days, respectively, and the separation
between them is ~700 days. We fit its spectral energy distribution (SED) and
analyze its light curves in the optical/UV, mid-infrared, and X-ray bands. We
find that this source can be interpreted as a two-phase scenario in which the
first phase is dominated by the stream circularization, and the second phase is
the delayed accretion. We use the succession of the self-crossing model and the
delayed accretion model to fit the first and the second peaks, respectively.
The fitting result implies that AT 2019avd can be interpreted by the partial
disruption of a 0.9 M_sun star by a 7 * 10^6 M_sun SMBH, but this result is
sensitive to the stellar model. Furthermore, we find the large-amplitude (by
factors up to $\sim 5$) X-ray variability in AT 2019avd can be interpreted as
the rigid-body precession of the misaligned disk due to the Lense--Thirring
effect of a spinning SMBH, with the precession period of 10 - 25 days.
