The object responsible for the mysterious brightening seen in 2006 (right) is ordinarily too dim to detect (left) (Image: Barbary et al.)

A wandering black hole may have torn apart a star to create a strange object that brightened mysteriously and then faded from view in 2006, a new study suggests. But more than three years later, astronomers are still at a loss to explain all the features of the strange event.

The object, called SCP 06F6, was first spotted in the constellation Bootes in February 2006 in a search for supernovae by the Hubble Space Telescope. The object flared to its maximum brightness over about 100 days, a period much longer than most supernovae, which do so in just 20 days.

Further analysis of the object’s spectrum in 2008 offered no more clues: SCP 06F6 seemed to resemble no known object, and astronomers couldn’t even say whether the event originated in the Milky Way or beyond.


Examining the work over coffee, Boris Gaensicke of the University of Warwick in Coventry, UK, and colleagues noticed that dips in the object’s light spectrum looked familiar. They resembled those created when light passes through a relatively cool area that is rich in carbon. “These wiggles are basically the fingerprints of carbon molecules,” Gaensicke says.

Too close

The expansion of space stretched these wavelengths of absorbed light to the redder part of the spectrum. The amount of the stretching suggests the object sits some 2 billion light years away.

Gaensicke and colleagues envision two scenarios that might explain the object. In one, a carbon-rich star gets too close to a middle- or heavy-weight black hole, which tears the star apart. Some of this material is absorbed by the black hole, and some is blasted away in a flare that was eventually seen from Earth as SCP 06F6.

Such flares brighten and dim with the same leisurely pace seen in SCP 06F6, and they also produce X-rays with a similar brightness to those the team found at the location of the firefly-like event.

Shock waves

But so far, no one has found a galaxy or even a cluster of stars in the same place as the object, which argues against the possibility. Although black holes might get gravitationally kicked out of their birthplaces in star clusters and galaxies during mergers with other black holes, they are more likely to be found in crowded areas.

“It would be very contrived to have a free-floating intermediate-mass black hole that would be disrupting a free-floating star in the intergalactic medium,” Gaensicke told New Scientist.

A more probable scenario is that the event is due to the explosive death of a massive star that was swaddled in carbon dust, Gaensicke says. The shock waves from the event would squeeze and heat the gas, producing an event that could be as prolonged as SCP 06F6.

But massive stars are also not likely to be found on their own. They live short lives, so should not have enough time to travel beyond their home galaxies. And the X-rays produced by SCP 06F6 were about 50 times brighter than those produced in the most luminous supernovae studied so far.

‘Best match’

The carbon spectral lines “are certainly the best match to the SCP 06F6 spectrum found so far”, but the wiggles are not a perfect fit, says Kyle Barbary of the University of California, Berkeley, lead author of the 2008 study on the object. “No possibility suggested so far is conclusive.”

Finding another example of the ‘firefly’ event would be the next big step in figuring out what the object is, Barbary says. “SCP 06F6 was found in a relatively small survey, so it is likely that there are a lot more of them out there,” Barbary told New Scientist. “I’m quite hopeful that we will be able to find out the true nature of the event in the near future.”

Gaensicke hopes one of Hubble’s new cameras, the Wide Field Camera 3, which was installed on the last space shuttle mission to visit the telescope, could reveal more about the object’s origins. The camera may be able to spot a host galaxy around the object that was too faint to see with other instruments.

Journal reference: The Astrophysical Journal Letters (vol 697, L129)