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Gamma-Ray Bursts
What causes gamma-ray bursts? The first burst was detected over 30
years ago and the mystery that surrounds their origin continues to
exist. We do know that gamma-ray bursts are the most energetic events
to occur in the Universe!
In order to understand what a
gamma-ray burst (or GRB) is, you must first realize that gamma-rays
are a type of light. In fact, gamma-rays are the most energetic form
of light known. Light is a form of energy called electromagnetic
radiation. Electromagnetic radiation comes in tiny packets of energy
called photons. Photons come in a wide range of
energies. Electromagnetic radiation can be placed in an arrangement
according to the energy amount of the photons. This orderly
arrangement is known as the electromagnetic spectrum.
At
the low-energy end of the spectrum we find radio waves. They have a
very long wavelength. At the high-energy end of the spectrum we find
gamma-rays. They possess a very short wavelength. For electromagnetic
waves, the relationship between wavelength and energy is an inverse
relationship. The shorter the wavelength, the greater the energy; the
longer the wavelength, the less the energy. Humans cannot see the
light forms at the low and high-energy ends of the spectrum. We can
only see light that falls in the visible range of the
spectrum. Visible light is in the middle of the spectrum and accounts
for a very small percentage of the energy range on the whole
spectrum.
If an astronomer were to study the Universe only in
the visible range of the spectrum, the large majority of events would
go unobserved. Cosmological events such as star birth and star death
emit photons that occur across the entire electromagnetic
spectrum. Thanks to considerable technological advances, astronomers
now have the ability to view the Universe in radio waves, gamma-rays,
and all energies in between. Distant quasars were first discovered by
the radio waves they emit. Galactic dust can be observed in the
infrared range while light from ordinary stars such as the Sun can be
observed in the visible and ultraviolet range. Extremely hot gas can
be observed by the X-rays that it emits. Observations in the gamma-ray
range of the spectrum reveal a very energetic Universe. Such energetic
phenomena as a blazar (which consist of a supermassive black hole with
jets of particles blasting away from near the event horizon), solar
flares, and the radioactive decay of atomic nuclei created in
supernova explosions all produce gamma-rays.
So what exactly is
a gamma-ray burst? At least once a day, the sky lights up with a
spectacular flash of gamma-rays coming from deep space (remember:
gamma-rays are not in the visible range of the electromagnetic
spectrum so we consequently are not aware of the phenomena). The
brightness of this flash of gamma-rays can temporarily overwhelm all
other gamma-ray sources in the Universe. The burst can last from a
fraction of a second to over a thousand seconds. The time that the
burst occurs and the direction from which it will come cannot be
predicted. Currently, the exact cause of these flashes is
unknown. Gamma-ray bursts can release more energy in 10 seconds than
the Sun will emit in its entire 10 billion-year lifetime. So far, it
appears that all of the bursts we have observed have come from outside
the Milky Way Galaxy. Scientists believe that a gamma-ray burst will
occur once every few million years here in the Milky Way, and in fact
may occur once every several hundred million years within a few
thousand light-years of Earth.
The first gamma-ray bursts were
detected while scientists were looking for violations of the Nuclear
Test Ban Treaty during the Cold War Era of the 1960s. Several
satellites employed to monitor treaty compliance detected a large
increase in the number of gamma-rays they counted each second. It was
determined that the gamma-rays were coming from outer space and not
from a nuclear bomb exploding in the Earth’s atmosphere. Although
Ray Klebesadel and his colleagues at the Los Alamos National
Laboratory in New Mexico found these bursts in data going back to
1967, their discovery was not reported to the world until 1973.
There are several theories currently discussed as possible causes
of gamma-ray bursts. One explanation proposes that they are the result
of colliding neutron stars. Neutron stars are the corpses of massive
stars (5 to 10 times the mass of our Sun) that have come to the ends
of their lifecycles. They are extremely dense. Although their diameter
may only be 20 kilometers, their mass is about 1.4 times that of the
Sun. A second theory proposes that gamma-ray bursts are the result of
a merging between a neutron star and a black hole or between two black
holes. Black holes result when supermassive (greater than 20 times the
mass of our Sun) stars die. A new theory that is attracting
considerable attention states that gamma-ray bursts occur as the
result of material shooting towards Earth at almost the speed of light
as the result of a hypernova. A hypernova explosion can occur when the
largest of the supermassive stars come to the end of their lives and
collapse to form black holes. Hypernova explosions can be at least 100
times more powerful than supernova explosions.
By solving the
mystery of gamma-ray bursts, scientists hope to gain further knowledge
of the origins of the Universe, the rate at which the Universe is
expanding, and the size of the Universe. Satellites such as
NASA’s Compton Gamma-Ray Observatory and Hubble Space Telescope,
and ESA’s BeppoSAX have given us valuable data in our quest to
solve the mystery of GRBs. These satellites have limitations,
however. One of them is that once a burst is detected, it takes too
long to reposition the satellite in order to face the burst and
collect data. They are also limited as to the range of the
electromagnetic spectrum in which they can make
observations. Recently, scientists were able to observe an optical
counterpart to a burst as the burst was occurring. This extraordinary
event occurred as the result of a great deal of planning, cooperation,
and luck. On January 23, 1999, a network of scientists was notified
within 4 seconds of the start of a burst that a burst was in
progress. Thanks to the Compton Gamma-Ray Observatory, BeppoSAX, the
Internet, and a special robotic ground-based telescope, scientists
were able to monitor the burst from start to finish at multiple
wavelengths. It had the optical brightness of 10 million billion Suns,
which was only one-thousandth of its gamma-ray brightness!
The
future looks good for solving the mystery of GRBs. A satellite called
the High Energy Transient Explorer (HETE) will be launched in late
1999 or early 2000. Its prime objective is to carry out a study of
gamma-ray bursts with X-ray and gamma-ray instruments. The original
HETE was lost due to a launch failure in 1996. The new high-energy
explorer is a similar satellite called HETE-2. Swift, a satellite with
the capacity to study the Universe in a multitude of wavelengths, has
been proposed for launch in approximately 2003. The satellite is aptly
named because once a burst is detected, it can be repositioned to face
the gamma ray source within 50 seconds. Through being able to
simultaneously observe the burst in the optical, ultraviolet, X-ray,
and gamma-ray ranges of the electromagnetic spectrum, scientists hope
to answer the many questions surrounding gamma-ray bursts. In
approximately 2005, the Gamma-Ray Large Area Space Telescope (GLAST)
will also be launched and should provide scientists with additional
insight into the gamma-ray burst mystery.
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