The UV/optical peak and X-ray brightening in TDE candidate AT2019azh: A
  case of stream-stream collision and delayed accretion
  We present and analyze the optical/UV and X-ray observations of a nearby
tidal disruption event (TDE) candidate AT2019azh, spanning from $\sim$ 30 d
before to $\sim$ 400 d after its early optical peak. The X-rays show a late
brightening by a factor of $\sim$ 30-100 around 200 days after discovery, while
the UV/opticals continuously decayed. The early X-rays show two flaring
episodes of variation, temporally uncorrelated with the early UV/opticals. We
found a clear sign of X-ray hardness evolution, i.e., the source is harder at
early times, and becomes softer as it brightens later. The drastically
different temporal behaviors in X-rays and UV/opticals suggest that the two
bands are physically distinct emission components, and probably arise from
different locations. These properties argue against the reprocessing of X-rays
by any outflow as the origin of the UV/optical peak. The full data are best
explained by a two-process scenario, in which the UV/optical peak is produced
by the debris stream-stream collisions during the circularization phase; some
shocked gas with low angular momentum forms an early, low-mass 'precursor'
accretion disk which emits the early X-rays. The major body of the disk is
formed after the circularization finishes, whose enhanced accretion rate
produces the late X-ray brightening. AT2019azh is a strong case of TDE whose
emission signatures of stream-stream collision and delayed accretion are both
identified.
