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

Research paper by Holoien, T. W.-; S, Kochanek, C. S, Prieto, J. L, Grupe, D, Chen, P, Godoy-;Rivera, D, Stanek, K. Z, Shappee, et al.

Indexed on: 16 Oct '16Published on: 07 Oct '16Published in: Monthly Notices of the Royal Astronomical Society



Abstract

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 x 1044 erg s–1 and radiated a total energy of E ~= 6.6 x 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 x 104 K to T ~ 4 x 104 K while the luminosity declines from L ~= 1.3 x 1044 erg s–1 to L ~= 2.3 x 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, $L\propto {\rm e}^{-(t-t_0)/\tau }$, with ~= 46.5 d for t 0 ~= 57241.6 (MJD), while a power-law decline of L  (t – t 0)–α with t 0 ~= 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.