File this one under: Things that go flash in the night.

While conducting a routine search for distant supernovae, astronomers observed a bright burst of light that they can’t account for. On Feb. 21, 2006, the Hubble Space Telescope first imaged the source of light, which continued to brighten over the next 100 days, peaked, and then finally faded to oblivion over another 100 days.

The time scale of brightening, as well as the particular characteristics of the colors of light seen, do not match any known astronomical phenomena.

“So far it’s unlike anything previously observed,” said Lawrence Berkeley National Laboratory astronomer Kyle Barbary during a press briefing Tuesday at the American Astronomical Society meeting in Long Beach, California.

While supernovae normally take three weeks to reach their peak brightness (or at most 70 days), this object, called SCP 06F6, took significantly longer. During the span that the light was visible, the team observed it with not only Hubble, but also with the Very Large Telescope in Chile and the Subaru Telescope and W.M. Keck Observatory in Hawaii.

The scientists can tell very little about the source of the flash — not even how far away it is, or how intrinsically bright. That means it could have come from some event in our own galaxy, or from some distant region of the universe. There is no visible star or galaxy at the site of flash to offer hints of what caused it.

Since the astronomers first announced the mystery sighting, many experts have offered guesses about what type of phenomenon could be behind it. Suggestions include a new type of supernova (such as the collapse and explosion of a unique star), a collision between a white dwarf star and a black hole, or even an exotic star made out of hypothetical “mirror matter.”

But none of the explanations are completely satisfying, Barbary said. “None of these are conclusive. There are sort of problems with each one that make none of them a sure bet.”

Some are even joking that it could have been caused by an alien civilization that had just turned on its own version of the Large Hadron Collider and blown itself up.

Since the object has completely faded away from sight by now, options for getting to the bottom of the mystery are slim.

“I think probably the only way to learn much more is to maybe find more of these things and look for similarities,” Barbary said.

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Image: NASA, ESA, and K. Barbary (University of California, Berkeley/Lawrence Berkeley National Lab, Supernova Cosmology Project)