Published online 19 September 2008 | Nature | doi:10.1038/news.2008.1122

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How they wonder what you are

The Hubble has caught an unprecedented celestial event in the act; but what act is it?

The Hubble Space TelescopeThe Hubble eyes a mysteryNASA

Astronomers are speculating over a mysterious burst of light detected by the Hubble Space Telescope.

The event, which bears only the name SCP 06F6, appeared in the telescope's field of view on 21 February 2006, brightened for around 100 days and then dimmed away to nothingness. There is no record of anything similar appearing before or since, and no clue as to what the source was; its spectrum appears unlike anything previously observed1.

"There's no obvious host galaxy or progenitor star," says Matt Mountain, director of the Space Telescope Science Institute in Baltimore, Maryland. "[The observers] have no idea what it is."

“We spent hours and days trying out all sorts of hypotheses.”

Saul Perlmutter
LBL

The observers were looking for supernovae, dying stars that give off a mighty flash of light as they explode. In recent years, astronomers have been using supernovae in distant galaxies to gauge the strength of dark energy, a mysterious force that appears to be expanding the Universe at an ever-accclerating rate.

The team found SCP 06F6 when sorting through a series of images taken in 2005 and 2006. The object slowly grew in brightness between February and May of 2006, and then dimmed over the next three months.

"We spent hours and days trying out all sorts of hypotheses," says Saul Perlmutter, an astrophysicist at Lawrence Berkeley National Laboratory in California who is leading the supernova study. But in the end, the team came up blank, he says.

Like a diamond in the sky

“It's not interstellar war — probably.”

Boris Gänsicke
University of Warwick

Other astronomers have been weighing in with their own theories. The spectral lines of the mysterious object could be those of molecular carbon, says Boris Gänsicke, an astronomer at the University of Warwick in Coventry, UK. On that basis he suggests that the flash might have been the explosion of a 'carbon star' around 2 billion light years away, well beyond the 'Local Group' of galaxies. The molecular lines correspond to a relatively low temperature, by star-death standards, of 5,000-6,000 kelvin, so Gänsicke suggests that the radiation was emitted either in a new kind of explosion or as the star was ripped apart by a black hole2.

But Gänsicke's group has also added to the mystery. The team found an observation of the same patch of sky made by the European X-ray satellite XMM Newton in early August of 2006 in which there appears to be an X-ray glow around the SCP 06F6. That would be difficult to explain given the supposed low temperature of the event. "These two facets together are difficult to explain," he says, adding "It's not interstellar war — probably."

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"This is a first-class puzzle," says Shri Kulkarni, an astronomer at the California Institute of Technology in Pasadena. Kulkarni himself speculates that the event might be a supernova-like event in which a star flared mightily but failed to explode. Still, he concedes that no one explanation for SCP 06F6 will prevail unless more data can be gathered from similar events: "You've got to find more clues and that means finding more objects."

Fortunately, Kulkarni says, there is a generation of telescopes being designed to use digital cameras and processing software to spot such transient events. The most impressive, known as the Large Synoptic Survey Telescope (LSST), will digitally image billions of stars and galaxies in search of unexpected events. It will survey more or less the whole visible sky every few nights, and its software will keep a constant watch for anything that changes from one sweep to the next. Who knows how many objects like, or unlike, SCP 06F6 such surveys will find, Kulkarni asks. "We are just on the verge of a revolution." 

  • References

    1. Barbary, K. et al. Preprint at http://arxiv.org/abs/0809.1648 (2008).
    2. Gänsicke, B. T. et al. Preprint at http://arxiv.org/abs/0809.2562 (2008).
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