An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

At 66-=Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≍10<SUP>6</SUP>-=M<SUB>⊙</SUB>, disrupting a star of ≍1-=M<SUB>⊙</SUB>. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t<SUP>2</SUP>, consistent with a photosphere expanding at constant velocity (≳2000-=km-=s<SUP>-1</SUP>), and a line-forming region producing initially blueshifted H and He-=II profiles with v = 3000-10-=000-=km-=s<SUP>-1</SUP>. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission - the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N-=III) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≍10<SUP>41</SUP>-=erg-=s<SUP>-1</SUP>. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.