November 28, 2004

SWIFT to Unravel Energetic Flashes

When the SWIFT detector looks at the sky using its gamma-ray sensitive view, the chance to unravel the cause of the most energetic stellar events may become possible.

Astrobiology Magazine -- Imagine an explosion that could emit a million times more energy than the combined output of all the stars in the Milky Way. If you could hear this burst, it would deafen you. If you could see one, it would blind you. Any life within a radius of many solar systems would be annihilated.

But just such an event happens a thousand times a day, lasts for about a minute, and then fades. Called gamma-ray bursts, these violent, distant explosions represent the greatest release of energy the universe has seen since the Big Bang. Satellites detect at least one of these bursts a day, and scientists do not know what causes them. Soon, however, a new satellite called SWIFT could offer more information about gamma-ray bursts (GRBs).

When SWIFT becomes operational in orbit this month, astronomers might finally be able to unravel some of the mysteries about gamma-ray bursts. SWIFT will find hundreds of these powerful flashes of energy each year.

Unlike visible light, gamma-rays are non-thermal, meaning that they are not produced in hot celestial bodies like the sun. Gamma rays occur in exceptional circumstances, such as in the aftermath of a stellar explosion, in the vicinity of black holes, or at the core of active galaxies.

Most charged particles in our solar system come from two sources: solar flares, which produce a rain of dangerous protons, and distant supernova explosions, which accelerate atomic nuclei --called "cosmic rays"-- to nearly light speed. Fortunately for life on Earth, a gamma particle from the universe does not penetrate to the Earth's surface.

Although this year marks the thirtieth anniversary of the discovery of gamma-ray bursts, much about them - such as why they happen - still remains mysterious. The first gamma-ray burst detection by the Air Force's Vela satellite dates to July 1967; however, because of security concerns, it wasn't until 1973 that Ray Klebesadel, Ian Strong and Roy Olson of Los Alamos National first announced the discovery of sixteen gamma-ray bursts.

In 1991, the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma-Ray Observatory discovered that this gamma-ray burst radiation was "isotropic," or uniformly coming from everywhere. BATSE could monitor nearly the entire sky for gamma-ray transient sources (i.e., sources that suddenly gave off a large amount of gamma-rays and then fade). In addition, BATSE could localize where in the sky the burst was coming from.

SWIFT will help scientists see the phenomenon from outside the Earth's atmosphere, but it may not track them any faster than the current High Energy Transient Experiment satellite [HETE]. HETE tries to locate a gamma-ray burst quickly and send tracking data to ground-based telescopes so they can catch the end of the GRB event before it disappears.

The prevailing explanation for this intense burst of radiation is that a collapsing star has burnt out its mass of fusionable energy, and thus announces its inevitable collapse into a black hole with a last, large explosion.

About 1,000 GRBs occur daily, but Earth-based telescopes only are able to see perhaps one a day Because SWIFT will be outside the Earth's atmosphere, it will see the GRB events even if the sky is cloudy or it's the middle of the day.


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