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 centre of 2MASX J20390918-3045201 (d similar or equal to 216 Mpc) by the All-Sky Automated Survey for SuperNovae. The source peaked at a bolometric luminosity of L similar or equal to 1.3 x 10(44) erg s(-1) and radiated a total energy of E similar or equal to 6.6 x 10(50) erg over the first similar to 3.5 months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from T similar to 2 x 10(4) K to T similar to 4 x 10(4) K while the luminosity declines from L similar or equal to 1.3 x 10(44) erg s(-1) to L similar or equal to 2.3 x 10(43) erg s(-1), requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline during this period is most consistent with an exponential decline, L proportional to e(-(t-t0)/tau), with tau similar or equal to 46.5 d for t(0) similar or equal to 57241.6 (MJD), while a power-law decline of L proportional to (t - t(0))(-alpha) with t(0) similar or equal to 57 212.3 and alpha = 1.62 provides a moderately worse fit. 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 similar to 3 months after discovery. The early spectroscopic features and colour evolution of ASASSN-15oi are consistent with a TDE, but the rapid spectral evolution is unique among optically selected TDEs.