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We present and analyze the optical/UV and X-ray observations of a nearbytidal disruption event (TDE) candidate AT2019azh, spanning from $\sim$ 30 dbefore to $\sim$ 400 d after its early optical peak. The X-rays show a latebrightening by a factor of $\sim$ 30-100 around 200 days after discovery, whilethe UV/opticals continuously decayed. The early X-rays show two flaringepisodes of variation, temporally uncorrelated with the early UV/opticals. Wefound a clear sign of X-ray hardness evolution, i.e., the source is harder atearly times, and becomes softer as it brightens later. The drasticallydifferent temporal behaviors in X-rays and UV/opticals suggest that the twobands are physically distinct emission components, and probably arise fromdifferent locations. These properties argue against the reprocessing of X-raysby any outflow as the origin of the UV/optical peak. The full data are bestexplained by a two-process scenario, in which the UV/optical peak is producedby the debris stream-stream collisions during the circularization phase; someshocked 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 isformed after the circularization finishes, whose enhanced accretion rateproduces the late X-ray brightening. AT2019azh is a strong case of TDE whoseemission signatures of stream-stream collision and delayed accretion are bothidentified.

The UV/optical peak and X-ray brightening in TDE candidate AT2019azh: A case of stream-stream collision and delayed accretion