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AT 2019azh is a H+He tidal disruption event (TDE) with one of the mostextensive ultraviolet and optical datasets available to date. We present ourphotometric and spectroscopic observations of this event starting several weeksbefore and out to approximately two years after g-band peak brightness andcombine them with public photometric data. This extensive dataset robustlyreveals a change in the light-curve slope and a bump in the rising light curveof a TDE for the first time, which may indicate more than one dominant emissionmechanism contributing to the pre-peak light curve. We further confirm therelation seen in previous TDEs whereby the redder emission peaks later than thebluer emission. The post-peak bolometric light curve of AT 2019azh is betterdescribed by an exponential decline than by the canonical t^{-5/3} (and in factany) power-law decline. We find a possible mid-infrared excess around peakoptical luminosity, but cannot determine its origin. In addition, we providethe earliest measurements of the Halpha emission-line evolution and find nosignificant time delay between the peak of the V-band light curve and that ofthe Halpha luminosity. These results can be used to constrain future models ofTDE line formation and emission mechanisms in general. More pre-peak 1-2 daycadence observations of TDEs are required to determine whether thecharacteristics observed here are common among TDEs. More importantly, detailedemission models are needed to fully exploit such observations for understandingthe emission physics of TDEs.

Light-Curve Structure and Halpha Line Formation in the Tidal Disruption Event AT 2019azh