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Abstract We present ultraviolet, optical, and near-infrared photometric and optical spectroscopic observations of the luminous fast blue optical transient (LFBOT) CSS 161010:045834–081803 (CSS 161010). The transient was found in a low-redshift (z= 0.033) dwarf galaxy. The light curves of CSS 161010 are characterized by an extremely fast evolution and blue colors. TheV-band light curve shows that CSS 161010 reaches an absolute peak of ${\mathit{M}}_V^{\max }=-20.66\pm 0.06$mag in 3.8 days from the start of the outburst. After maximum, CSS 161010 follows a power-law decline ∝t^−2.8±0.1in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf, and AT 2020xnd. However, unlike these objects, the spectra of CSS 161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines with velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS 161010 are unique features not seen in any transient before CSS 161010. The combined observational properties of CSS 161010 and itsM_*∼ 10⁸M_⊙dwarf galaxy host favor the tidal disruption of a star by an intermediate-mass black hole as its origin. 

Abstract AT 2019azh is a H+He tidal disruption event (TDE) with one of the most extensive ultraviolet and optical data sets available to date. We present our photometric and spectroscopic observations of this event starting several weeks before and out to approximately 2 yr after theg-band's peak brightness and combine them with public photometric data. This extensive data set robustly reveals a change in the light-curve slope and a possible bump in the rising light curve of a TDE for the first time, which may indicate more than one dominant emission mechanism contributing to the pre-peak light curve. Indeed, we find that theMOSFiT-derived parameters of AT 2019azh, which assume reprocessed accretion as the sole source of emission, are not entirely self-consistent. We further confirm the relation seen in previous TDEs whereby the redder emission peaks later than the bluer emission. The post-peak bolometric light curve of AT 2019azh is better described by an exponential decline than by the canonicalt^−5/3(and in fact any) power-law decline. We find a possible mid-infrared excess around the peak optical luminosity, but cannot determine its origin. In addition, we provide the earliest measurements of the Hαemission-line evolution and find no significant time delay between the peak of theV-band light curve and that of the Hαluminosity. These results can be used to constrain future models of TDE line formation and emission mechanisms in general. More pre-peak 1–2 days cadence observations of TDEs are required to determine whether the characteristics observed here are common among TDEs. More importantly, detailed emission models are needed to fully exploit such observations for understanding the emission physics of TDEs.