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

Abstract

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 ($d\simeq 216$ Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source peaked at a bolometric luminosity of $L\simeq 1.3\times 10^{44}$ ergs s${}^{-1}$ and radiated a total energy of $E\simeq 6.6\times 10^{50}$ ergs over the first $\sim 3.5$ months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from $T\sim 2\times 10^4$ K to $T\sim 4\times 10^4$ K while the luminosity declines from $L\simeq 1.3\times 10^{44}$ ergs s${}^{-1}$ to $L\simeq 2.3\times 10^{43}$ ergs s${}^{-1}$, requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline during this period is most consistent with an exponential decline, $L\propto e^{-(t-t_0)/\ddot{I}„}$, with $\ddot{I}„\simeq 46.5$ days for $t_0\simeq 57241.6$ (MJD), while a power-law decline of $L\propto (t-t_0{)}^{-\hat{I}\pm }$ with $t_0\simeq 57212.3$ and $\hat{I}\pm =1.62$ 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 $\sim 3$ 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.

18 pages, 13 figures, 5 tables. Photometric data presented in this submission are included as ancillary files. Manuscript updated to reflect changes made in the published version. For a brief video explaining this paper, see https://youtu.be/clYXbqAQ0u0