[tt] Space 'firefly' resembles no known object

[Eugen Leitl]

http://space.newscientist.com/article/dn14738-space-firefly-resembles-no-known-object.html

Space 'firefly' resembles no known object

    * 00:06 16 September 2008

    * Maggie McKee

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

An object that brightened intensely and then faded back into obscurity over a
period of about seven months is unlike anything astronomers have seen before,
a new study reports.

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.

Nothing had been seen at its location before it started to brighten, and
nothing was spotted after it dimmed. That suggests it is normally too faint
to observe and that it brightened by at least 120 times during its
firefly-like episode.

Stars are known to brighten dramatically when they explode as supernovae. But
supernovae reach their maximum brightness after about 20 days, and this
object took a leisurely 100 days to hit its peak.

The object's spectrum is also bizarre. It does not match that of anything
seen in the mammoth Sloan Digital Sky Survey, which has mapped more than a
quarter of the sky.  Near or far

The spectrum shows a handful of spectral lines, but when astronomers try to
trace any one of them to an element – such as magnesium, the other lines fail
to match up with known elements.

"Because we can't see anything we recognise in the spectrum, we can't tell if
it's even in the galaxy or in another galaxy," says Kyle Barbary of the
University of California, Berkeley, lead author of the new study.

If it's inside the galaxy, it might be a dim stellar ember called a white
dwarf. White dwarfs can brighten suddenly when they steal matter from a
nearby stellar companion or suck in matter from a disc of debris around them.

But that process of sucking in matter would have to happen in a "strange way"
to explain the odd spectrum, Barbary says: "It would still leave something
unanswered."

If the object lies outside the galaxy, explaining its provenance is no
easier.

When its discovery was first reported in 2006, astronomer Stefan Immler of
NASA's Goddard Space Flight Center in Greenbelt, Maryland, suggested to New
Scientist that the object might be a very distant supernova, lying about 12
billion light years away.

Bell curve

If it was that distant, the expansion of the universe would relativistically
draw out a supernova explosion, making a 20-day event stretch out over a
period of 100 days.

But Barbary says that idea is unlikely for two reasons. He and colleagues
believe the object lies no farther than 11 billion light years away.

"If the transient were more distant than this, we would see signs of
absorption from intergalactic hydrogen in the blue part of the spectrum," he
told New Scientist. A supernova lying 11 billion light years away would cause
a 20-day brightening event to stretch over just 70 days – a month shy of
explaining the observations.

Secondly, observations show that the object's brightness faded at the same
rate as it had initially risen – creating a symmetrical, bell-shaped "light
curve". Supernovae typically have asymmetrical light curves – they fade much
more slowly than they brighten.

'All bets are off'

"When I first heard about the 100-day rise about two years ago, I thought
that this could be a distant supernova," Immler told New Scientist. "However,
now that more results are in, the object is more enigmatic than ever."

"The biggest problem is that the distance is not known," Immler says. "If you
can't figure out the distance, you are in trouble as an astronomer and all
bets are off."

The team may try to re-observe the same region of space, just in case the
object flares up again. "It's possible that it's a cyclical thing and would
come back, but it would be good to know if it doesn't," Barbary says.

"At this point, there are a few possibilities that we have ruled out, but no
possibilities that look particularly promising," he told New Scientist. "We
are hoping someone else might have seen something similar or might be able to
shed some light on it."

Journal reference: Astrophysical Journal (forthcoming)