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GAMMA-RAY
BURSTS LIGHT THE WAY TO THE EARLY UNIVERSE
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| NASA
astronomers say they have uncovered a specific property
of gamma-ray bursts that will enable them to gauge the
distances to thousands of these powerful explosions, many
perhaps beyond the reach of all existing telescopes. |
This
finding, experts say, may allow scientists to determine the
geometry of the Universe throughout its various epochs, as
well as when and where massive stars formed in the very early
Universe.
A
team led by Dr. Jay Norris, an astrophysicist at NASA's Goddard
Space Flight Center in Greenbelt, Md., performed the new analysis
using data from NASA's Compton Gamma Ray Observatory and several
optical telescopes.
"If our finding holds up, this could be a new window
on the distant Universe," said Norris. "Many gamma-ray
bursts can be detected beyond the farthest supernovae and
quasars we can now see."
Gamma-ray bursts occur randomly several times a day without
warning, typically last only a few seconds to a minute, and
apparently release more energy than any explosions in the
Universe other than the Big Bang itself.
Norris found that, in a single burst, gamma rays of different
energies reached the Earth-orbiting detectors at slightly
different times, with the higher-energy gamma rays arriving
before the lower-energy gamma rays. The amount of lag time
between the two corresponded to the burst's estimated peak
luminosity and distance. The lag was shorter for the more
luminous bursts.
The new work was reported at the Fifth Huntsville Gamma-Ray
Burst Symposium in Huntsville, Alabama, on October 19, and
has been accepted for publication to The Astrophysical Journal.
Related findings, derived independently by Dr. Edward Fenimore
of Los Alamos National Laboratory and also reported to the
Huntsville conference, lend confidence to the new result,
astronomers say.
Gamma-ray bursts were discovered in the late 1960s, but only
recently have most astronomers agreed that a large fraction
of the bursts originate in the very distant, early Universe.
The bursts fade quickly at gamma-ray energies and are hard
to pinpoint, making it difficult to observe a burst's optical
afterglow and determine a distance, or redshift.
Redshift is a common measurement of astronomical distances.
The more distant an object is from Earth, the faster it is
receding due to the expansion of the Universe, and the greater
its light is stretched or redshifted. This is similar to the
way a siren on an ambulance appears to drop in pitch as the
ambulance speeds away. Objects at high redshifts serve as
probes to the early Universe, for their light has taken billions
of years to reach Earth.
Yet of the thousands of gamma-ray bursts detected, fewer than
ten have had an afterglow or host galaxy whose redshift could
be determined with optical telescopes. This new finding by
Goddard scientists has the potential of gauging the distances
of many bursts from gamma-ray data alone.
Comparing the intrinsic burst luminosity (the actual brightness
regardless of distance, as measured by redshifts and now,
perhaps, by photon lag times) with the measured luminosity
(how bright the burst appears to Earth-orbiting gamma-ray
detectors) yields a distance to the source.
GAMMA
RAY BURST EXPLOSION
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| Click
on the image above see an animated GIF file of a gamma
ray burst. (This is a rather large file, so it may
take a minute to load.) |
One
of many explanations for gamma ray bursts is a hypernova,
an exploding star a hundred times more powerful that a typical
exploding star, called a supernova.
After a massive red-giant star exhausts its fuel, its heart
of iron is crushed under its own weight. Its iron core collapses
until it becomes black hole, essentially a distortion in space
where the gravity is so powerful that near it, nothing, not
even light, can escape. The middle layers of the star spiral
into the black hole, heating up and causing a tremendous explosion.
Evidence exists that jets of material are ejected at almost
the speed of light during the initial phase of the explosion
by a poorly understood process. The explosion then rips through
the outer layers of the star, blasting them into space.
As the fireball expands and cools, the light it emits becomes
progressively less energetic; from gamma rays to X-rays, then
to ultraviolet and visible light, and finally down to low
energy microwave and radio. It thus becomes visible to other
kinds of telescopes sensitive to these other types of light.
One
of many explanations for gamma ray bursts is a hypernova,
an exploding star a hundred times more powerful that a typical
exploding star, called a supernova.
After a massive red-giant star exhausts its fuel, its heart
of iron is crushed under its own weight. Its iron core collapses
until it becomes black hole, essentially a distortion in space
where the gravity is so powerful that near it, nothing, not
even light, can escape. The middle layers of the star spiral
into the black hole, heating up and causing a tremendous explosion.
Evidence exists that jets of material are ejected at almost
the speed of light during the initial phase of the explosion
by a poorly understood process. The explosion then rips through
the outer layers of the star, blasting them into space.
As the fireball expands and cools, the light it emits becomes
progressively less energetic; from gamma rays to X-rays, then
to ultraviolet and visible light, and finally down to low
energy microwave and radio. It thus becomes visible to other
kinds of telescopes sensitive to these other types of light.
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For
animated movies,
go to:
NOTE:
These are large files and take time to load, please be patient.
Low
resolution thumbnail images below. Click on thumbnail
for a larger picture.
For
TIFF resolution of the above images, go to: http://www.gsfc.nasa.gov/ftp/pub/gamray
GAMMA
RAY BURSTS IN THE SKY
Gamma
ray bursts occur randomly several times a day without warning,
last only a few seconds to a minute, and release more energy
than any event in the Universe other than the Big Bang. About
six have known distances; all of which place them at very
remote regions in the cosmos. NASA scientists recently discovered
a way to determine the true brightness, regardless of distance,
for any gamma-ray burst.
By comparing the bursts' true brightness to how bright they
appear, astronomers can determine how far away they really
are. With the distance information, the bursts' extreme brightness
will let them be used as cosmic beacons to locate where and
when star-birth regions formed, even if the newly born stars
and galaxies can't yet be seen with present telescopes.
For
more information about gamma ray bursts, visit the following
links:
http://universe.gsfc.nasa.gov/press/images/GRB
What are gamma rays?
http://imagine.gsfc.nasa.gov/docs/science/know_l1
/emspectrum.html
What
is a gamma ray burst, and what do we know so far?
http://imagine.gsfc.nasa.gov/docs/introduction/bursts.html
More
on gamma ray bursts:
http://www.batse.com/
Spacecraft hunting for gamma ray bursts:
The
Compton Gamma Ray Observatory:
http://cossc.gsfc.nasa.gov/cossc/PR.html
The
Rossi X-ray Timing Explorer:
http://heasarc/docs/heasarc/missions.html
BeppoSax:
http://www.sdc.asi.it/
The
Hubble Space Telescope finds the home of a gamma ray burst:
http://oposite.stsci.edu/pubinfo/pr/1998/17/
Future
missions will take a closer look
The
Gamma ray Large Area Space Telescope:
http://glast.gsfc.nasa.gov/
Swift:
http://swift.gsfc.nasa.gov/
High
Energy Transient Explorer:
http://space.mit.edu/HETE/
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