Space Forum / Astronomy / January 2005

NASA Tracks Three Space Bursts, Says Stellar Explosions Imminent

By Robert Roy Britt
Senior Science Writer

Three powerful bursts of energy from different regions of space could
presage spectacular explosions of huge stars, astronomers just
announced.

The eruptions are likely imminent.

Scientists around the world are scrambling to track the blasts, NASA
officials said last night. There is no danger to Earth from the
expected stellar explosions, called supernovas.

Yet never before have astronomers had such advance warning of the
faraway explosions. In fact, they don't even know if their forecasts
are right.

What is clear is that as the flashes develop into explosions -- or not
-- knowledge of how stars die is likely to grow.

'Beautiful' bursts

A blast of X-rays was spotted Sept. 12, and another on Sept. 16. Each
came from a different location in the sky and from galaxies far beyond
our own. A more powerful eruption was detected Sept. 24 from yet
another spot in the sky. This third flash, importantly, was on the
verge between an X-ray eruption and a more energetic gamma-ray burst,
which involves a more powerful form of radiation.

X-rays and gamma rays are types of light, just like less powerful
visible light and lowly radio waves. All are part of the
electromagnetic spectrum.

The three high-energy flashes were each discovered by NASA's orbiting
High-Energy Transient Explorer (HETE- 2) observatory. There is no
reason to suspect there's any connection between the three blasts.

"We think it's just a strange coincidence," George Ricker, of the
Massachusetts Institute of Technology, said in a telephone interview
today.

Telescopes around the world have since raced to track each event.

"Each burst has been beautiful," Ricker said. "Depending on how these
evolve, they could support important theories about supernova[s] and
gamma-ray bursts."

Ricker told SPACE.com the stars will likely go supernova 10 to 20 days
after the initial bursts that were spotted.

The initial events have faded beyond the visibility of small
professional telescopes and are now being monitored by some of the
world's largest ground-based observatories. Backyard astronomers
likely could not find the bursts, Ricker said.

Head-scratchers

Gamma-ray bursts are the most energetic events in the universe other
than the Big Bang. They briefly outshine entire galaxies. Astronomers
think each burst is related to the explosion of a very massive star
that has used up its main fuel. Much material is blasted into space,
and some falls back rapidly and collapses into a tiny sphere more
dense than most folks can imagine, resulting in the formation of a
black hole.

In some cases, however, the energy might be unleashed when two black
holes collide.

But experts are not sure why some supernovas are accompanied by
gamma-ray bursts and others seem to shoot out only X-rays (the latter
assumption has not even been convincingly determined). The leading
theory is that when a star collapses after exploding, it sends out two
incredibly swift jets of material, one along each of its poles. If a
jet is pointed toward Earth, the thinking goes, we see a gamma-ray
burst. Otherwise we note only the X rays.

Other theorists argue that gamma-ray bursts and X-ray flashes are
different animals altogether.

All this could become much clearer in coming days as the three new
eruptions are monitored by a global telescope network designed to
detect each of the different wavelengths of energy involved.

Nature on a rampage

The eruptions are all probably a billion or so light-year away, Ricker
said. That's relatively close in comparison to most gamma-ray bursts,
which may explain why the X-ray flashes have been seen at all.

"These past two weeks have been like 'cock, fire, reload,'" Ricker
said. "Nature keeps on delivering."

Until recently, the events leading up to gamma-ray bursts and black
hole formation had not been seen.

The bursts are known to come routinely from every direction in the
sky. But they last just seconds, sometimes less than a second, so in
most cases only the aftermath is witnessed. Astronomers hope this time
they've seen the prelude and can witness the entire process.

Observations of other events in recent years linked gamma-ray bursts
to supernovas. Now, follow-up observations of the Sept. 24 blast,
named GRB040924, suggests X-rays and gamma rays do indeed emanate from
the same event.

The recent bursts "may be the first time we see an X-ray flash lead to
a supernova," said theorist Stanford Woosley of the University of
California at Santa Cruz.