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We present ground-based and Swift photometric and spectroscopic observationsof the tidal disruption event (TDE) ASASSN-15oi, discovered at the center of2MASX J20390918-3045201 ($d\simeq216$ Mpc) by the All-Sky Automated Survey forSuperNovae (ASAS-SN). The source peaked at a bolometric luminosity of$L\simeq1.3\times10^{44}$ ergs s$^{-1}$ and radiated a total energy of$E\simeq6.6\times10^{50}$ ergs over the first $\sim3.5$ months of observations.The early optical/UV emission of the source can be fit by a blackbody withtemperature increasing from $T\sim2\times10^4$ K to $T\sim4\times10^4$ K whilethe luminosity declines from $L\simeq1.3\times10^{44}$ ergs s$^{-1}$ to$L\simeq2.3\times10^{43}$ ergs s$^{-1}$, requiring the photosphere to beshrinking rapidly. The optical/UV luminosity decline during this period is mostconsistent with an exponential decline, $L\propto e^{-(t-t_0)/\tau}$, with$\tau \simeq46.5$ days for $t_0\simeq57241.6$ (MJD), while a power-law declineof $L\propto (t-t_0)^{-\alpha}$ with $t_0\simeq57212.3$ and $\alpha=1.62$provides a moderately worse fit. ASASSN-15oi also exhibits roughly constantsoft X-ray emission that is significantly weaker than the optical/UV emission.Spectra of the source show broad helium emission lines and strong bluecontinuum emission in early epochs, although these features fade rapidly andare not present $\sim3$ months after discovery. The early spectroscopicfeatures and color evolution of ASASSN-15oi are consistent with a TDE, but therapid spectral evolution is unique among optically-selected TDEs.

ASASSN-15oi: A Rapidly Evolving, Luminous Tidal Disruption Event at 216 Mpc