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Oct.
22, 2008: Gamma-ray bursts are by far the brightest
and most powerful explosions in the Universe, second only
to the Big Bang itself. So it might seem a bit surprising
that a group of them has gone missing.
A
single gamma-ray burst (GRB) can easily outshine an entire
galaxy containing hundreds of billions of stars. Powerful
telescopes can see them from clear across the Universe. And
because the deeper you look into space, the farther back in
time you see, astronomers should be able to see GRBs from
the time when the very first stars were forming after the
Big Bang.
Yet
they don't. Gamma-ray bursts from that early epoch seem to
be missing, and astronomers are wondering where they are.
Above:
An artist's concept of the first stars forming after the Big
Bang. Credit: NASA. [Larger
image]
"This
is one of the biggest questions in the gamma-ray business,"
says astrophysicist Neil Gehrels of the Goddard Space Flight
Center. "It's something we're going to be talking a lot
about today at the GRB Symposium."
Gehrels
has joined about a hundred of his colleagues from 25 countries
for the 6th Huntsville Gamma-ray Burst Symposium underway this
week in Huntsville, Alabama. Missing gamma-ray bursts are one
of the top mysteries on the
agenda.
Until
recently, experts were grappling with an even more fundamental
question about GRBs: what the heck are they? Astronomers had
observed these astonishing bursts since the 1960s, but nobody
could imagine an event powerful enough to create them.
The
answer eventually came from Stan Woosley, a theoretical astrophysicist
at the University of California in San Diego. He suggested
that when young, supermassive stars with low metal content
collapse under their own weight to form black holes, the stars'
rotation funnels the explosive energy into two streamlined
jets that shoot out from the stars' poles, like the axis of
a gyro. We only see the burst if one of these two jets happens
to be pointed toward Earth. The concentration of energy into
narrow jets is why GRBs that we do observe appear so remarkably
bright.
 Right:
The collapsar model of gamma-ray bursts. Click on the image
to view a 5 MB animation.
Note:
Woosley's"collapsar model" explains the common long
gamma-ray burst, explosions lasting 2 seconds or more. The
cause of another class of shorter-lived GRBs is still a mystery,
but that's another
story.
The
first waves of star formation after the Big Bang should have
produced plenty of metal-poor supermassive stars ripe for
collapse. If true, GRBs from that epoch should be abundant.
So where are they?
One
possibility is they're not missing at all.
"Part
of the problem is that burst profiles get stretched out by
the expansion of the Universe, so it is harder to recognize
them as bursts in the first place," explains astrophysicist
Lynn Cominsky of Sonoma State University. "The bursts
could be happening, but we're not noticing them."
Another
trouble is the afterglow—the fading debris that tells so much
about a burst, including its distance. "Afterglows from
the most distant GRBs may be too red shifted to be seen by
current generations of telescopes," she notes.
 "Red
shift" is how much the wavelength of light is stretched
when it travels to us across the expanding Universe. The farther
away a thing is, the more its light is stretched, and the
greater the red shift. Until recently, the largest red shift
ever measured for a GRB was 6.3. Then, last month, Gehrels
and colleagues using NASA's Swift satellite found one with
a red shift of 6.7 or 12.8 billion light years away. So far,
that's the record.
Right:
The X-ray afterglow of a gamma-ray burst at red shift 6.7:
full
story.
"Gamma-ray
bursts are predicted in the red shift range 10 to 20, but
so far no one has seen anything beyond 6.7," says Cominsky.
The
luminous afterglow of such distant bursts would be red shifted
all the way into the infrared. "There's a huge effort
right now to try to get those infrared observations,"
Gehrels says, but in the meantime it's difficult to verify
whether a candidate 7+ GRB is truly that far away.
As
infrared telescopes improve, scientists should eventually
be able to measure the distance to GRBs with red shifts greater
than 7 — if they exist. And that's a big IF. What if the missing
GRBs really are missing?
"That
would teach us something very interesting about the Universe,"
says Gehrels.
The
Sixth Huntsville Gamma-Ray Burst Symposium 2008 is sponsored
by NASA's Fermi and Swift Projects and hosted by the Fermi
GBM Team based at the Marshall Space Flight Center in Huntsville.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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