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GAMMA-RAY
BURSTS LIGHT THE WAY TO THE EARLY UNIVERSE
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1 | | 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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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. Back
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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: GRB
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8 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/ Back
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