We report the discovery and early evolution of ASASSN-15lh, the most luminous supernova ever found. At redshift z=0.2326, ASASSN-15lh reached an absolute magnitude of M_{u,AB} ~ -23.5 and bolometric luminosity L_bol ~ 2.2x10^45 ergs/s, which is >~ 2 times more luminous than any previously known supernova. Its spectra match the hydrogen-poor sub-class of super-luminous supernovae (SLSNe-I), whose energy sources and progenitors are poorly understood. In contrast to known SLSNe-I, most of which reside in star-forming, dwarf galaxies, its host appears to be a luminous galaxy (M_V ~ -22; M_K ~ -25.1) with little star formation. In the two months since its first detection, ASASSN-15lh has radiated ~7.5x10^51 ergs, challenging the popular magnetar model for the engine of SLSNe-I.
Authors: Subo Dong (KIAA-PKU), B. J. Shappee (Carnegie), J. L. Prieto (UDP), S. W. Jha (Rutgers), K. Z. Stanek (Ohio State), T. W.-S. Holoien (Ohio State), C. S. Kochanek (Ohio State), T. A. Thompson (Ohio State), N. Morrell (LCO), I. B. Thompson (Carnegie), U. Basu, J. F. Beacom, D. Bersier, J. Brimacombe, J. S. Brown, Ping Chen, E. Conseil, A. B. Danilet, E. Falco, D. Grupe, S. Kiyota, G. Masi, B. Nicholls, F. Olivares, G. Pignata, G. Pojmanski, G. V. Simonian, D. M. Szczygiel, P. R. Wozniak
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