Astronomical mystery: Hubble discovers unusual transient phenomenon

 Under the title Discovery of an Unusual Optical Transient with the Hubble Space Telescope an international group of 30 scientists presented just on September 8, a 5 pages report detailing their discovery.

The abstract says:

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 ~100 days, reached a peak magnitude of ~21.0 in both i775 and z850, and then declined over a similar time scale.
The transient’s spectrum, in addition to being inconsistent with all known supernova
types, is not matched to any spectrum in the Sloan Digital Sky Survey (SDSS) database. We suggest that the transient may be one of a new class.

The complete document in PDF could be seen at: 

http://supernova.lbl.gov/2006Transient/figures/barbary2008.pdf