ASASSN-15oi: a rapidly evolving, luminous tidal disruption event at 216 Mpc

We present ground-based and Swift photometric and spectroscopic observations of the tidal disruption event (TDE) ASASSN-15oi, discovered at the center of 2MASX J20390918-3045201 ($d\\simeq216$ Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source peaked at a bolometric luminosity of $L\\simeq1.9\\times10^{44}$ ergs s$^{-1}$ and radiated a total energy of $E\\simeq5.0\\times10^{50}$ ergs over the $\\sim3.5$ months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from $T\\sim2\\times10^4$ K to $T\\sim6\\times10^4$ K while the luminosity declines from $L\\simeq1.9\\times10^{44}$ ergs s$^{-1}$ to $L\\simeq2.8\\times10^{43}$ ergs s$^{-1}$, requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline is broadly consistent with an exponential decline, $L\\propto e^{-t/t_0}$, with $t_0\\simeq35$ days. ASASSN-15oi also exhibits roughly constant soft X-ray emission that is significantly weaker than the optical/UV emission. Spectra of the source show broad helium emission lines and strong blue continuum emission in early epochs, although these features fade rapidly and are not present $\\sim3$ months after discovery. The early spectroscopic features and color evolution of ASASSN-15oi are consistent with a TDE, but the rapid spectral evolution is unique among optically-selected TDEs.