We report the discovery by the Robotic Optical Transient Search Experiment ( ROTSE-IIIb ) telescope of SN 2008es , an overluminous supernova ( SN ) at z = 0.205 with a peak visual magnitude of - 22.2 .
We present multiwavelength follow-up observations with the Swift satellite and several ground-based optical telescopes .
The ROTSE-IIIb observations constrain the time of explosion to be 23 \pm 1 rest-frame days before maximum .
The linear decay of the optical light curve , and the combination of a symmetric , broad H \alpha emission line profile with broad P Cygni H \beta and Na I \lambda 5892 profiles , are properties reminiscent of the bright Type II-L SNe 1979C and 1980K , although SN 2008es is greater than 10 times more luminous .
The host galaxy is undetected in pre-supernova Sloan Digital Sky Survey images , and similar to Type II-L SN 2005ap ( the most luminous SN ever observed ) , the host is most likely a dwarf galaxy with M _ { r } > -17 . Swift Ultraviolet/Optical Telescope observations in combination with Palomar 60 inch photometry measure the spectral energy distribution of the SN from 200 to 800 nm to be a blackbody that cools from 14,000 K at the time of the optical peak to 6400 K 65 days later .
The inferred blackbody radius is in good agreement with the radius expected for the expansion speed measured from the broad lines ( 10,000 km s ^ { -1 } ) .
The bolometric luminosity at the optical peak is 2.8 \times 10 ^ { 44 } erg s ^ { -1 } , with a total energy radiated over the next 65 days of 5.6 \times 10 ^ { 50 } erg .
The exceptional luminosity of SN 2008es requires an efficient conversion of kinetic energy produced from the core-collapse explosion into radiation .
We favor a model in which the large peak luminosity is a consequence of the core-collapse of a progenitor star with a low-mass extended hydrogen envelope and a stellar wind with a density close to the upper limit on the mass-loss rate measured from the lack of an X-ray detection by the Swift X-Ray Telescope .