- Saturday, 23 January, 1999, is a day
most astronomers will not forget. At 0947 GMT a burst of gamma rays, radiation
of the highest energy, flashed through our Solar System.
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- It had been produced by a cosmic cataclysm
near the edge of the Universe and had been travelling through space for
billions of years.
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- Some of the gamma rays hit our planet
causing no harm - most passed by on their never-ending journey through
the cosmos.
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- Some of gamma rays were picked up by
two orbiting satellites with instruments set to detect them. Signals from
the satellites triggered an automatic Internet alert. Within 22 seconds,
an optical telescope in New Mexico swung towards the position in the sky
from which the burst had come.
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- The analysis of the data collected has
now been published in a group of papers published in the leading journals
Nature and Science.
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- Neutron stars
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- Astronomers detect gamma-ray bursters
almost every day. They believe these flashes of intense radiation are caused
when two, super-dense stars, called neutron stars, collide.
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- Neutron stars are small, only about 25
km across, but a teaspoonful of neutron star material would weigh a billion
tonnes. Some speculate that a gamma burst signifies the violent birth of
a black hole.
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- When the neutron stars collide there
is a titanic explosion - one that literally ripples to the other end of
the Universe.
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- This burst was named GRB 990123. It was
far more powerful than anything that had ever been seen before. In fact,
it was too powerful to be explained easily by current theories.
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- In the Science and Nature journals, astronomers
have tried to come up with some explanations. One would seem to be that
the burst was amplified, beamed towards us and just happened to appear
brighter than it really was.
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- Current theories
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- "The burst appeared to be more luminous
than the whole of the rest of the Universe, and that would be very hard
to explain by most current theories," said California Institute of
Technology Professor Shrinivas Kulkarni.
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- Caltech Professor S. George Djorgovski
said: "It was ten times more luminous than the brightest burst seen
so far, and that was quite unexpected.
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- "If the gamma-rays were emitted
equally in all directions, their energy would correspond to ten thousand
times the energy emitted by our Sun over its entire lifetime so far, which
is about 5 billion years. Yet the burst lasted only a few tens of seconds."
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- Since that January day, astronomers all
over the world have used many telescopes and satellite observatories to
watch the burst's afterglow. They have also tried to work out from where
the burst came.
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- The distance to the burst has been determined
from its spectrum, and was found to be about 9 billion light years from
the Earth.
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- Host galaxy
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- "We were stunned," said Prof
Djorgovski.
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- "This was much further than we expected,
and together with the observed brightness of the burst it implied an incredible
luminosity.
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- "The peak brightness of the visible
light afterglow alone would be millions of times higher than a luminosity
of an entire galaxy, and thousands of times brighter than the most luminous
quasars known."
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- As the burst faded, astronomers managed
to find its host galaxy. It appears different to our own galaxy, but not
that unusual and contains many hot, blue, young stars.
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- A detailed study suggests the jet of
energy came from an explosion moving close to the speed of light, and pointing
nearly towards the Earth. This would have amplified its brightness and
may explain its seemingly enormous brightness.
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