SN 2006gy: Discovery of the Most Luminous Supernova Ever Recorded, Powered by the Death of an Extremely Massive Star like η Carinae

(abridged) We report our discovery and observations of the peculiar Type IIn\nsupernova SN2006gy in NGC1260, revealing that it reached a peak magnitude of\n-22, making it the most luminous supernova ever recorded. It is not yet clear\nwhat powers the total radiated energy of 1e51 erg, but we argue that any\nmechanism -- thermal emission, circumstellar interaction, or 56Ni decay --\nrequires a very massive progenitor star. The circumstellar interaction\nhypothesis would require truly exceptional conditions around the star probably\nexperienced an LBV eruption like the 19th century eruption of eta Carinae.\nAlternatively, radioactive decay of 56Ni may be a less objectionable\nhypothesis. That power source would imply a large Ni mass of 22 Msun, requiring\nthat SN2006gy was a pair-instability supernova where the star's core was\nobliterated. SN2006gy is the first supernova for which we have good reason to\nsuspect a pair-instability explosion. Based on a number of lines of evidence,\nwe rule out the hypothesis that SN 2006gy was a ``Type IIa'' event. Instead, we\npropose that the progenitor may have been a very massive evolved object like\neta Carinae that, contrary to expectations, failed to completely shed its\nmassive hydrogen envelope before it died. Our interpretation of SN2006gy\nimplies that the most massive stars can explode earlier than expected, during\nthe LBV phase, preventing them from ever becoming Wolf-Rayet stars. SN2006gy\nalso suggests that the most massive stars can create brilliant supernovae\ninstead of dying ignominious deaths through direct collapse to a black hole.\n