An astronomical enigma has been spotted by a team hunting for very distant supernovas for their studies of the early universe.
At first glance, the object discovered on 22 February in the constellation Bootes resembled an ordinary supernova. But it kept growing brighter for much too long, and its spectrum was abnormal.
The mysterious object was spotted by the Hubble Space Telescope's Advanced Camera for Surveys and took at least 100 days to reach peak brightness, says Kyle Dawson of the Lawrence Berkeley National Laboratory in California, US, a member of the Supernova Cosmology Project. Normal supernovas reach peak brightness about 20 days after the blast.
Hubble saw nothing on 29 January at the point in the sky where the object appeared, so it must have brightened by more than a factor of 200. It has just begun to fade.
The object's spectrum is also unusual. The researchers could find no matches when they compared it with objects in the wide-ranging Sloan Digital Sky Survey. And its colour has not changed since it was first observed. Normally, temperature changes after an explosion cause colour changes.
How far away the object is, as determined by its redshift, is uncertain. If the strongest feature in the spectrum is a pair of calcium absorption lines, its red shift would be 0.54, corresponding to a distance of 5.5 billion light years.
But the object is at least one magnitude brighter than a Type 1A supernova would be at that distance, Dawson told New Scientist. And there is no sign of a host galaxy, which should be visible.
Astronomers can only speculate on what the object is. "It could be some galactic variable [star], a supernova or a quasar. But none of those makes any sense," Dawson says.
The object's behaviour doesn't match any known quasar. The team is not convinced the object is outside our galaxy, but nothing like it is known inside the galaxy. Furthermore, the region of Bootes is a largely empty area of the sky far from the plane of the Milky Way.
"It's a very intriguing object," says supernova researcher Stefan Immler of the NASA Goddard Space Flight Center in Maryland, US, but he will not rule out the possibility that it might be a supernova.
If it was extremely distant, the expansion of the Universe would relativistically stretch a supernova explosion. We would see a 20-day event stretched to 100 days at a red shift of 4, corresponding to an object about 12 billion light years away seen just 1.5 billion years after the big bang.
That would require an extremely bright supernova, but Immler says that such young stars would explode differently because they contain fewer heavy elements than modern stars.
The best hope to resolve the question is to make more observations, and so Dawson has booked time for 25 June. "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 recognise," he says. Observations outside the visible spectrum may also provide more insights.