These enigmatic flashes, called gamma-ray bursts, are the most powerful explosions known in the Universe, emitting more than one hundred billion times the energy than the Sun does in an entire year. Yet they last only a few milliseconds to a few minutes, never to appear in the same spot again.

The Swift satellite is named for the nimble bird, because it can swiftly turn and point its instruments to catch a burst "on the fly" to study both the burst and its afterglow. The afterglow phenomenon follows the initial gamma-ray flash in most bursts; and it can linger in X-ray light, optical light and radio waves for hours to weeks, providing great detail.

"Gamma-ray bursts have ranked among the biggest mysteries in astronomy since their discovery over 35 years ago," said Dr. Neil Gehrels, Swift Lead Scientist from NASA's Goddard Space Flight Center in Greenbelt, Md.

"Swift is just the right tool needed to solve this mystery. One of Swift's instruments will detect the burst, while, within a minute, two higher-resolution telescopes will be swung around for an in-depth look. Meanwhile, Swift will 'e-mail' scientists and telescopes around the world to observe the burst in real-time."

The Burst Alert Telescope (BAT) instrument, built by NASA Goddard, will detect and locate about two gamma-ray bursts per week, relaying a 1- to 4-arc-minute position to the ground within about 20 seconds.

This position will then be used to "swiftly" re-point the satellite to bring the burst area into the narrower fields of view to study the afterglow with the X-ray Telescope (XRT) and the UltraViolet/Optical Telescope (UVOT).

These two longer-wavelength (lower-energy) instruments will determine an arc-second position of a burst and the spectrum of its afterglow at visible to x-ray wavelengths. For most of the bursts detected with Swift this data, together with observations conducted with ground-based telescopes, will enable measurement of the redshift, or distance, to the burst source.

The afterglow provides crucial information about the dynamics of the burst, but scientists need precise information about the burst in order to locate the afterglow.

Swift notifies the community�which includes museums and the general public, along with scientists at world-class observatories�via the Goddard-maintained Gamma-ray Burst Coordinates Network (GCN). A network of dedicated ground-based robotic telescopes distributed around the world await Swift-GCN alerts.

Continuous burst information flows through the Swift Mission Operations Center, located at Penn State. Penn State, a key U.S. collaborator, built the XRT with University of Leicester (UK) and the Astronomical Observatory of Brera (Italy) and the UVOT with Mullard Space Science Lab (UK).

In addition to providing new clues to the nature of the burst mechanism, Swift's detection of gamma-ray bursts could provide a bonanza of cosmological data.

"Some bursts likely originate from the farthest reaches, and hence earliest epoch, of the Universe," said Swift Mission Director John Nousek, professor of astronomy and astrophysics at Penn State. "They act like beacons shining through everything along their paths, including the gas between and within galaxies along the line of sight."

Theorists have suggested that some bursts may originate from the first generation of stars, and Swift's unprecedented sensitivity will provide the first opportunity to test this hypothesis.

With NASA's High-Energy Transient Explorer (HETE-2), now in operation, scientists have determined that at least some bursts involve the explosions of massive stars. Swift will fine-tune this knowledge�that is, answer such questions as how massive, how far, what kind of host galaxies, and why are some bursts so different from others?

While the link between some fraction of bursts with the death of massive stars appears firm, others may signal the merger of neutron stars or black holes orbiting each other in exotic binary star systems.

Swift will determine whether there are different classes of gamma-ray bursts associated with a particular origin scenario. Swift may be fast enough to identify afterglows from short bursts, if they exist. Afterglows have only been seen for bursts lasting longer than two seconds. "We may be seeing only half the story so far," said Gehrels.

The Swift team expects to detect and analyze over 100 bursts a year. When not catching gamma-ray bursts, Swift will conduct an all-sky survey at high-energy "hard" X-ray wavelengths, which will be 20 times more sensitive than previous measurements. Scientists expect that Swift's enhanced sensitivity relative to earlier surveys will uncover over 400 new supermassive black holes.

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