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Swift: the satellite that's always on call: this acrobatic spacecraft is shedding new light on nature's biggest explosions.

Since their discovery in the 1960s, gamma-ray bursts (GRBs) have been touted as among the biggest mysteries in modern astronomy. These megaexplosions pack as much as 1052 ergs--more energy than our Sun could produce at its present rate in 80 billion years. GRBs light up every corner of the universe and then disappear without a trace, occasionally within seconds.

Early studies revealed two distinct types of GRBs: longduration bursts lasting more than 2 seconds and short, millisecond bursts that, until recently, evaded scrutiny. A link between long-duration bursts and supernova-like stellar explosions was forged in 2003 (S&T: March 2004, page 32), but the short bursts remained a complete mystery. Now, though, astronomers finally think they know what causes these enigmatic blasts, and they mainly have one satellite to thank: the Swift Gamma Ray Burst Explorer.

Just halfway into its two-year mission, NASA's high-energy space observatory already has proved worthy of its flighty name and relatively low sticker price of $180 million. Joining the still-active, 5-year-old High Energy Transient Explorer 2 (HETE-2), which localizes an average of 25 bursts a year, Swift was turned on in December 2004, and its GRB locations came pouring in twice weekly.

In one of the only fields of astronomy where seconds count, the secret to Swift's success is its ability to instantaneously slew to a GRB at almost any point on the sky. Swift's Burst Alert Telescope sees a seventh of the entire sky at one time. Once it detects a GRB, Swift can rapidly pinpoint that explosion within 20 seconds. It then slews to the burst and swiftly scrutinizes its X-ray afterglow. Finally, Swift's ultraviolet and visible-light telescope provides a detailed optical light curve and a spectrum. The spectrum can determine the burst's redshift and hence its distance.

The dichotomy of GRB characteristics has led astronomers down separate paths to uncover the mechanisms that trigger each kind of burst. Long GRBs were fairly well understood before Swift's launch. Their bright afterglows continue for days, sometimes weeks, allowing groundbased telescopes to localize and identify their sources: massive, black-hole-spawning supernovae. These hypernovae (or collapsars) are dying stars containing 40 times the mass of the Sun. When their cores collapse, such stars produce explosions that are 10 times more energetic than a typical supernova. Most of this energy shoots out of the erupting star at 99.999 percent of the speed of light in the form of magnetically driven polar jets (top image). We see a powerful flash of gamma rays if the jets point earthward, and an afterglow at longer wavelengths if the blast energizes surrounding gas.

Short GRBs--those lasting anywhere from one millisecond to 2 seconds--were the real prize for Swift. These elusive bursts fade quickly, leaving little time to pinpoint their locations. They are theorized to be the last gasp of energy expelled when two neutron stars (or perhaps a neutron star and a black hole, as depicted in the three illustrations above) merge after orbiting one another for millions or billions of years. When they collide, the paired compact objects will form a single black hole in a few hundredths of a second.

Catching a short-duration burst seconds after it goes off is just what Swift was designed to do, and on May 9, 2005, a 0.03-second GRB provided the opportunity (S&T: August 2005, page 16). The burst's afterglow faded after only 5 minutes, but not before Swift had a chance to record a rapidly fading X-ray source and, for the first time, to pinpoint the position precisely enough for ground-based telescopes to identify its host galaxy. Not surprisingly, the May 9th event took place within the outer fringes of an old elliptical galaxy located 2.7 billion light-years away (at redshift 0.226). No one would expect a hypernova--the explosive death of a rapidly evolving, and therefore very young, star--in an elliptical galaxy, with its uniformly ancient stellar demographics.

Even after Swift caught this milestone burst, the prevailing binary-merger theory for subsecond GRBs still needed more evidence to hold up to scientific scrutiny. Fortunately, three more short-burst afterglows were detected last summer, one by the HETE-2 instrument and two others by Swift (S&T: December 2005, page 21). These events all had cosmologically modest redshifts (from 0.16 to 0.722) and occurred near older galaxies that presumably contain many compact-object binary systems. "As recently as last summer, we didn't know the distance scale of short GRBs," says Caltech astronomer Joshua Bloom. "It's quite remarkable how fast we got the answer when you consider that we've been asking the question for 30 years."

By responding to GRBs faster than any previous instrument, Swift has also dug up a new challenge to tackle. At gamma-ray and X-ray energies, GRB light curves are complex, often flaring many times (see the bottom graph at left). Particularly prevalent in long bursts, these gamma-ray and X-ray "aftershocks" occur after the initial blast but before the traditional afterglow of the surrounding material. "Now we see light-curve spikes and flares in about a third of the bursts," says Swift's principal investigator, Neil Gehrels (NASA/Goddard Space Flight Center).

At first, these hiccups suggested that clumps of gas were suddenly glowing, as if they had gotten in the path of the jet and were consumed by the explosion. "But that doesn't seem to be the answer," explains Gehrels; "the flares are happening too soon." Rather, the time scale of the chaotic variations suggests that the aftershocks are taking place not in the surrounding material but within the central engine itself.

"Swift has been a blessing and a hindrance," says Pennsylvania State University theorist Peter Meszaros. "The GRBs we are detecting are twice as distant, so that's good, but it's harder to pinpoint the location." This blessing manifested itself last September when Swift observed the farthest GRB ever seen (see the images below). It shattered the distance record with a redshift of 6.29--placing this event only about 900 million years after the Big Bang (the previous record holder had a redshift of 4.5). Only a handful of quasars have been found farther than redshift 6.

Theorists expect that the first stars were made of almost pure hydrogen and helium and tended to be extremely massive. When these behemoths went supernova, they emitted long-duration GRBs that Swift has the sensitivity to detect. Studying such distant bursts may provide clues to early cosmic eras that are accessible no other way.

Long-duration bursts may have been understood before Swift appeared on the scene, but now they can be used to their fullest. "Gamma-ray bursts are a new lighthouse to studying the distant universe," says Bloom. Ultimately Swift is expected to detect blasts at redshifts of 10 or more. It seems only a matter of time before Swift will lead the way to an understanding of how the first stars and galaxies formed.

Swift at a Glance

NAME: Swift Gamma Ray Burst Explorer

LAUNCH: November 20, 2004

ORBIT: Circular, 600-kilometer (373-mile) altitude

TELESCOPES: Burst Alert Telescope (BAT) detecting initial bursts; X-Ray Telescope (XRT) studying X-ray afterglows; 30-centimeter f/12.7 modified Ritchey-Chretien Ultraviolet/Optical Telescope (UVOT) obtaining light curves and spectra


DURATION: Funded for 2 years; 7-year life expectancy


* Determine the origins of gamma-ray bursts (GRBs)

* Classify and search for new types of GRBs

* Determine how GRB blast waves evolve and interact with their surroundings

* Use GRBs to study the early universe

* Perform a sensitive gamma-ray survey of the sky

Assistant editor Lisa R. Johnston carefully monitors all bursts of high energy in the Boston music scene.
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Author:Johnston, Lisa R.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2006
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