Discovery of the Ultraluminous Type IIn Supernova PS1-11vo

We report the discovery by the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1 or PS1) of PS1-11vo, a Type IIn supernova (SN) at z = 0.116 with a peak r-band absolute magnitude of M = -20.4. We also present optical spectroscopic observations from the Apache Point Observatory Echelle Spectrograph, the Gemini Multi-Object Spectrograph, and the MMT Blue Channel Spectrograph over a period of roughly 1 year after detection. PS1-11vo is one of the longest lived, most luminous supernovae (SNe) ever discovered and the highest quality SN IIn documented by Pan-STARRS1. The Pan-STARRS1 photometric observations indicate maximum was reached roughly 50 days after the time of explosion, during which the SN rose by approximately 5 mag. Spectra of PS1-11vo display a prominent hydrogen alpha emission line and P Cygni profile, typical of SNe IIn. We compare the photometric and spectroscopic observations of PS1-11vo to those of other SNe II, including several recent ultraluminous SNe IIn. Finally, we examine its spectral energy distribution to model various parameters of the SN and its host environment at 5 days past maximum, estimating a peak luminosity of L = 4.5E43 erg/s and an initial Nickel-56 mass of 4.5 solar masses. This work is supported in part by the NSF REU and DOD ASSURE programs under NSF grant no. 0754568 and by the Smithsonian Institution