Supernova hunters are boggled by the mysterious object they spotted back in February in the constellation Bootes. At the time, it resembled an ordinary supernova, but since then it's evolved into a much brighter object with an abnormal spectrum.

Discovered by Hubble Space Telescope's Advanced Camera for Survey, the enigmatic object took at least 100 days to reach its peak brightness, a strange occurrence as a normal supernova usually reaches its brightness peak about 20 days after the blast.

The object's unusual spectrum also baffled researchers who couldn't find any matches in the Sloan Digital Sky Survey. Its colour is unchanged since it was first spotted. Another deviation from the normal supernova-forming process, as temperature changes after an explosion cause changes in color.

Even the distance of the object has not been determined, though its pair of calcium absorption lines indicates that its red shift would be 0.54, which translates to a distance of 5.5 billion light years. According to Kyle Dawson of the Lawrence Berkeley National Laboratory, a member of the Supernova Cosmology Project, the object is at least one magnitude brighter than a Type 1A supernova would be at that distance. Furthermore there's no sign of a host galaxy, which should've been visible.

Stefan Immler of the NASA Goddard Space Flight Center is not ruling out the possibility that it might be a supernova, though he calls it "a very intriguing object".

Dawson can only speculate, "It could be some galactic variable [star], a supernova or a quasar." But none of those makes any sense. Puzzling as the object may be, Dawson and his team have booked June 25 for another round of observations. "It's still going to be visible for another 2.5 months on the ground. We hope the spectrum will evolve and we see some features we can recognize."