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 ≃ 216 Mpc) by the All-Sky Automated Survey for SuperNovae. The source peaked at a bolometric luminosity of L ≃ 1.3 × 10⁴⁴ erg s^-1 and radiated a total energy of E ≃ 6.6 × 10⁵⁰ erg over the first ∼3.5 months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from T ∼ 2 × 10⁴ K to T ∼ 4 × 10⁴ K while the luminosity declines from L ≃ 1.3 × 10⁴⁴ erg s^-1 to L ≃ 2.3 × 10⁴³ 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∝ e^{-(t-t_0)/τ}, with τ ≃ 46.5 d for t₀ ≃ 57241.6 (MJD), while a power-law decline of L ∝ (t - t₀)^-α with t₀ ≃ 57 212.3 and α = 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 ∼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.