ASASSN-15lh: A highly super-luminous supernova

Abstract

We report the discovery of ASASSN-15lh (SN 2015L), which we interpret as the most luminous supernova yet found. At redshift z = 0.2326, ASASSN-15lh reached an absolute magnitude of M<SUB>u</SUB><SUB>,AB</SUB> = -23.5 ± 0.1 and bolometric luminosity L<SUB>bol</SUB> = (2.2 ± 0.2) × 10<SUP>45</SUP> ergs s<SUP>-1</SUP>, which is more than twice as luminous as any previously known supernova. It has several major features characteristic of the hydrogen-poor super-luminous supernovae (SLSNe-I), whose energy sources and progenitors are currently poorly understood. In contrast to most previously known SLSNe-I that reside in star-forming dwarf galaxies, ASASSN-15lh appears to be hosted by a luminous galaxy (M<SUB>K</SUB> ≈ -25.5) with little star formation. In the 4 months since first detection, ASASSN-15lh radiated (1.1 ± 0.2) × 10<SUP>52</SUP> ergs, challenging the magnetar model for its engine.