DISCOVERY OF AN UNUSUAL OPTICAL TRANSIENT WITH THE HUBBLE SPACE TELESCOPE

We present observations of SCP 06F6, an unusual optical transient discovered during the Hubble Space Telescope Cluster Supernova Survey. The transient brightened over a period of similar to 100 days, reached a peak magnitude of similar to 21.0 in both i(775) and z(850), and then declined over a similar timescale. There is no host galaxy or progenitor star detected at the location of the transient to a 3 sigma upper limit of i(775) >= 26.4 and z(850) >= 26.1, giving a corresponding lower limit on the flux increase of a factor of similar to 120. Multiple spectra show five broad absorption bands between 4100 angstrom and 6500 angstrom, and a mostly featureless continuum longward of 6500 angstrom. The shape of the light curve is inconsistent with microlensing. The transient's spectrum, in addition to being inconsistent with all known supernova types, does not match any spectrum in the Sloan Digital Sky Survey database. We suggest that the transient may be one of a new class.