The UV/Optical Peak and X-Ray Brightening in TDE Candidate AT 2019azh: A Case of Stream–Stream Collision and Delayed Accretion

Abstract

We present and analyze the optical/UV and X-ray observations of a nearby tidal disruption event (TDE) candidate, AT 2019azh, from ∼30 days before to ∼400 days after its early optical peak. The X-rays show a late brightening by a factor of ∼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 that 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. Event AT 2019azh is a strong case of a TDE whose emission signatures of stream–stream collision and delayed accretion are both identified.