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

Research paper by T. W. -S. Holoien, C. S. Kochanek, J. L. Prieto, D. Grupe, Ping Chen, D. Godoy-Rivera, K. Z. Stanek, B. J. Shappee, Subo Dong, J. S. Brown, U. Basu, J. F. Beacom, D. Bersier, J. Brimacombe, E. K. Carlson, et al.

Indexed on: 02 Feb '16Published on: 02 Feb '16Published in: arXiv - Astrophysics - High Energy Astrophysical Phenomena



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 ($d\simeq216$ 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.