Six months ago, satellite telescopes
spotted an exceptionally bright burst of energy that would have been the most
distant object in the universe ever visible to the naked eye, if anyone saw it.
Even though no humans have reported
seeing it directly, the gamma-ray burst, an explosion that signals the violent
death of a massive star, is changing theories of how these events look. Gamma
ray bursts are typically accompanied by intense releases of other forms of
radiation, from X-rays to visible light.
This burst, dubbed GRB 080319B, was first
detected by the Swift satellite on March 19, while the spacecraft was
serendipitously looking at another gamma-ray burst in the same area of the sky.
The light it emitted in the visible
part of the spectrum was so intense that the burst would have been visible to
the naked eye in the constellation Bootes for about
40 seconds — no other gamma-ray burst has ever been visible without a telescope.
The incredible amount of energy
given off across the entire electromagnetic spectrum during a gamma-ray burst
is what Jonathan Grindlay of the Harvard-Smithsonian
Center
for Astrophysics calls "the birth pangs of a black hole. This is the
scream."
It took the light of GRB 080319B
about 7.4 billion years to reach Earth, placing the explosion "more than
halfway back to the Big Bang and the origin of our universe," Grindlay wrote in an editorial accompanying a new study
of the burst in the Sept. 10 issue of the journal Nature.
This means that the explosion
happened 3 billion years before the sun or Earth even formed, Grindlay added. When astronomers see such distant objects,
they are in effect looking
back in time.
After the Swift detection,
telescopes around the world were alerted and trained their eyes onto the new
gamma-ray burst, giving scientists a highly detailed view of these explosions —
the most luminous in the universe — whose formation and structure still hold
many mysteries. The findings in the new study of GRB 080319B challenge some of
the commonly-held views of gamma-ray bursts.
A violent death and birth
Gamma-ray bursts are something of an
extreme form of supernovas,
the bright explosions that mark the deaths of massive stars.
But possibly one in every 1,000
supernovae is not one of these "normal" explosions; instead of the
star simply dying, its core collapses to form a black hole — that event
generates a gamma-ray burst. (Just what the conditions are that boost a normal
supernova into a gamma-ray burst are not known.)
The gamma-ray burst is actually a
powerful jet of material sent out by the spinning accretion disk that surrounds
a newborn black hole. This bright part of the burst typically only lasts
between 3 and 100 seconds, and an afterglow follows that can last for days or
weeks.
GRB 080319B's jet was one of the
brightest ever observed in terms of gamma rays, and it was unusually bright in
optical wavelengths.
"It was unexpected that it was
this bright," said lead author of the new study Judith Racusin, a graduate
student at Penn State University.
Narrow jet
The study suggests that the jet of
the gamma-burst
actually has two components: a narrow, ultra-fast jet at the core of a wider,
slightly slower jet.
The narrow part of the jet
of GRB 080319B was so fast that it shot material directly toward Earth at
99.99995 percent the speed of light.
Scientists think that it was because
the jet was pointed straight at us that it appeared so much brighter than
previously-observed gamma-ray bursts. The researchers speculate that it is rare
to detect the inner core of the jet because it is so narrow — only about 1/100th
the size of the full moon as seen from Earth.
For this reason, astronomers think
they may have missed the narrow core of the jet in previous bursts: "We're
primarily just seeing the outer jet," Grindlay
told SPACE.com, because we are not seeing those bursts head-on.
Racusin said that it isn't know for
sure that all gamma-ray bursts have this two component structure to their jets,
but that the theory fits what they saw with GRB 080319B.
To see a similarly bright burst,
astronomers would have to catch another one aimed directly at Earth, which
Racusin and her colleagues calculated should happen about once ever three to 10
years.
Swift may not still be around in 10
years, but the recently launched GLAST satellite (recently renamed
Fermi) and other missions in the planning stages could catch a glimpse of
them.
But whether or not they do, Racusin
knows one thing: With GRB 080319B, "we got lucky."
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