First Responder

David Pacchioli
September 21, 2010
Swift satellite launching into outer space

First responder: After five remarkable years of discovery, the Swift satellite hasrewritten the book on the tremendous deep-space explosions known as gamma-ray bursts.

"They are the mightiest powerhouses in the Universe: they burn up as much energy in a few seconds as the Sun does in ten billion years." That's how Péter Mészáros describes gamma-ray bursts.

These fleeting astronomical events, blinding flashes from the deepest reaches of space, were first observed in 1967, by U.S. Vela satellites deployed to check for nuclear test ban violations by the Soviet Union. Yet what came into view as a Cold War artifact long remained a celestial enigma. What could cause these seemingly random explosions? Just how powerful were they? Were they close to us or far?

When the Vela data were finally declassified in 1973, the interest generated among astronomers was immediate. Over the next two decades, thousands of papers were written, proffering hundreds of theories. But the sheer strangeness of these blasts, as well as their transience, made them resistant to study.

Mészáros, director of Penn State's Center for Particle Astrophysics, got interested in gamma-ray bursts in 1991, when images from NASA's Compton Observatory revealed that these high-energy flashes mapped isotropically—i.e., uniformly—across the sky. The fact that they didn't appear to be bunched along the plane of our galaxy, Mészáros explains, meant that the bursts had to be either extremely close to Earth or, more likely, very, very distant. If the latter, judging by their relative brightness, they would have to be "stupendously" energetic, the biggest explosions since the Big Bang. "This," he remembers, "was the beginning of real understanding."

two researchers in dress shirts posing in front of a computer monitor
Frederic Weber

Peter Mészáros (left) and Derek Fox

During a sabbatical at Cambridge University, Mészáros began working with the well-known British cosmologist Martin Rees, and the two developed what is still the working model for how such explosions may occur. At the time, Mészáros has written, "gamma-ray bursts were thought to be just that, bursts of gamma rays which were largely devoid of any observable traces at any other wavelengths" of the electromagnetic spectrum. Yet in February 1997, he and Rees published a paper arguing that the rapid high-energy bursts should be followed by lingering emissions at longer wavelengths. Less than three weeks later, their prediction was confirmed when the Italian-Dutch satellite Beppo-SAX picked up fading X-ray signals coming from a number of gamma-ray events. Unlike the gamma rays themselves, these "afterglows" hung around for hours or days or even weeks, long enough that their distances from Earth could be readily measured. And they proved indeed to be billions of light years away, at the farthest edges of the cosmos.

The Need for Speed

The Compton data confirmed that gamma-ray bursts, as John Nousek puts it, "come in two flavors." So-called long bursts, the type that left afterglows, last anywhere from two seconds to several minutes, explains Nousek, professor of astrophysics at Penn State. Short ones tend to burn out in less than a second, some as little as a tenth of a second.

Aided by afterglows, researchers could not only fix the location of long bursts, but also search their host galaxies for the objects that may have caused them. As the evidence mounted, it became apparent that most of these bursts were associated with supernovae, the violent death throes of massive young stars 30 times the size of the Sun. When such a star comes to the end of its nuclear fuel, astronomers posit, its core collapses to form a black hole. As more stellar material is drawn into the greedy vortex, jets of plasma are ejected from either end of the hole at nearly the speed of light. In rare cases, it seems, these jets emit the intense flash of gamma rays. "Basically," says Derek Fox, another Penn State astronomer, "a long-duration gamma-ray burst is a very unusual type of supernova."

David Burrows and John Nousek pose for camera

David Burrows (left) and John Nousek

That's the theory. The shorter bursts, however, didn't fit the mold. While hugely energetic themselves, they are about ten times less so than long-duration bursts. And their extreme abruptness doesn't match the time required for a massive star's demise. What made astronomers particularly uncomfortable, though, was that a short burst was over so fast it couldn't actually be studied. With existing telescopes it was impossible to react quickly enough to get a bead on the thing before it vanished, so short bursts remained a mystery. "What was needed was a rapid-response system that could catch these fleeting events as they were happening," remembers Nousek. Under NASA's direction, an international consortium from the United States, Italy, and the United Kingdom took on the challenge of designing such a thing, and that's how the Swift Gamma Ray Observatory was born.

Early Successes

Swift was conceived as a multi-wavelength observatory; its three telescopes would operate in tandem. The Burst Alert Telescope, or BAT, would continuously scan large swaths of sky for unusual flashes in the gamma ray range. When it picked up a burst, the spacecraft would rotate itself within seconds to train X-ray and optical telescopes on the event. The emissions at these longer wavelengths, the thinking was, would allow Swift to quickly fix the burst's position while simultaneously relaying that information to more powerful follow-up telescopes on the ground.

As lead university partner, Penn State was charged with designing and building the X-ray and optical instruments (known as the XRT and UVOT, respectively), and, once Swift was launched, with controlling its activity from a Mission Operations Center in State College. A large cadre of Penn State astronomers would be involved, including Nousek at the head the operations staff, Mészáros as lead scientist, David Burrows to spearhead the XRT, and Pete Roming as chief of the UVOT.

Swift satellite launch

Swift's November 2004 launch from Cape Canaveral

Swift was launched from Cape Canaveral on November 20, 2004, and within six months its rapid-response capability had paid off. On May 9, 2005, Swift's BAT detected a short-duration burst, and the XRT and UVOT kicked in on cue to pick up its afterglow, the first one ever captured. When a second short burst was spotted in early July, Penn State's Fox led a team that followed up with both ground and space-based telescopes to fix its distance from Earth and get a bead on its host galaxy. In a set of papers published in the journal Nature, Fox and his co-authors presented their findings, concluding that short bursts were an entirely different animal. Rather than from the explosions of massive stars, they appeared to result from collisions between two much smaller, denser star remnants—called neutron stars—that were drawn together into a kind of death spiral.

"A neutron star is the mass of the Sun extremely compressed into a chunk of rock the size of Manhattan," Fox explained at the time. "The collision of two objects that dense results in the formation of a new black hole and creates short, powerful bursts of gamma rays that last only a few milliseconds but are a trillion times brighter than our Sun." A NASA news conference called it the end of a 35-year-old mystery.

This confirmation of the so-called merger model, Swift's first big coup, was soon followed by success of another sort. In September 2005, the satellite detected a powerful explosion at the edge of the visible universe. The unusually long burst—it lasted approximately 500 seconds, over eight minutes—was determined to have occurred some 12.7 billion light years from Earth, which made it the most distant explosion ever seen. Only one other celestial object—a quasar—had ever been discovered at a greater distance.

Targets of Opportunity

After such a start, it's not surprising that within two years of launch, Swift had accomplished virtually everything its makers had promised. "There have been numerous highlights," Mészáros acknowledges, "but perhaps the most valuable thing is that it has made possible a sort of production-line measurement of bursts. Before Swift we knew the distances and host galaxies of maybe 30 bursts. Now we're at 150, heading up to 200. Once you start accumulating that type of statistic, you can really start classifying things."

So much so, Nousek says, that "we've now moved on into at least second- and possibly third-generation gamma ray burst astronomy. As a result of the continuing availability of bursts, a lot of ground-based and even space-based correlative studies have come about. It's even catalyzed development of new instrumentation."

artists depiction of Swift satellite
Sonoma State University, NASA E/PO Aurore Simonnet

Artist's depiction of Swift satellite

In addition, the last few years have seen a conscious broadening of Swift's mission. "After we had discovered all the things we said we would," Nousek explains, "the question became 'how do we use Swift when we're not looking at gamma ray bursts?'" Ensconced in its lower-Earth orbit, the satellite makes a fresh revolution every 96 minutes, during which it can handle three or four targets. "Sometimes there are no gamma ray bursts visible," he says. But Swift's versatility and quickness make it a natural for spotting comets, variable stars, and other transient objects. "When we advertised these other uses, what we call targets of opportunity, the response was enthusiastic. It's become so popular that we're now getting several requests a day."

One such request resulted in a truly serendipitous discovery when Swift chanced to catch a "normal" supernova—not a gamma-ray variant—in the act of exploding. As David Burrows explains, the satellite just happened to have turned its eye onto a particular patch of sky at the precise moment when an ordinary star went kaflooey, something never before witnessed. "The whole thing only lasted about five minutes," he says. "It was phenomenal luck. But it has given us unique insights into the physics of these explosions."

Far Out

In the spring of 2009, Swift made news around the world with yet another major find. It began as a routine spotting of a middling, 10-second gamma ray burst by the satellite's BAT. After a quick pivot, the XRT picked up an afterglow in X-ray light, "just like hundreds of others that we have seen," according to Burrows. The UVOT saw nothing in the visual range—again, this wasn't unusual. But when follow-ups by infrared telescopes on the ground began reporting that they had seen the fuzzy remnant, says Pete Roming, "we had our first clue that this was a very distant object."

Soon a team led by Fox, analyzing data from the Gemini North Telescope on Mauna Kea, Hawaii, had arrived at a figure of slightly over 13 billion light-years, a jaw-dropping distance whose announcement set off a frenzied race among competing teams of astronomers seeking to confirm it. When the dust finally settled, GRB 090423, as it is known, turned out to be the most distant object that has ever been seen—dramatic evidence of an explosion that occurred just a few hundred million years after the Big Bang, during that scarce-imaginable era when stars and galaxies were first forming.

To astronomers, Mészáros explains, a bright object at that distance is above all a time machine. "A gamma ray burst is a very intense light source," he says. "As that light shines through the universe it's like a searchlight cutting through fog. You see the little clouds of fog which are close to the source and then closer to you, so you get a cross-section of what kinds of materials were present—what kinds of atoms, what was the abundance of carbon versus hydrogen at different times. You can count how many proto-galaxies formed when the universe was five percent of its present age, or 10 percent. It's a very sensitive probe."

The prospect of reaching back even farther toward the Big Bang, suddenly made realistic by Swift's find, has inspired Fox, Burrows, and Roming to take the lead in developing a next-generation satellite called JANUS, for Joint Astrophysics Nascent Universe Satellite. If funded, they say, it will feature the same rapid-response capability that has made Swift so valuable, but unlike Swift, the all-purpose workhorse, it will be trained exclusively on the most luminous objects—both gamma ray bursts and quasars—at the very infancy of time.

More Miles to Run

This past April, the Swift satellite reached a major milestone. "By catching 500 gamma-ray bursts ' on the fly' and studying them in unprecedented detail," Burrows was quoted in a press release, "Swift has given us a much deeper understanding of these elusive explosions and their role in shaping our Universe."

To put that achievement into context: Gamma ray bursts are occurring somewhere in the sky at a rate of about two a day, and of that steady stream Swift's BAT pulls in about one in eight for detailed study. Of all the bursts whose distances have ever been measured, Swift—in orbit for just shy of six years—is responsible for 75 percent of them. "Before Swift," says Nousek simply, "the field was very much data-starved."

Although it has already well-outlived its minimum life expectancy as a three-year mission, he is quick to add, Swift is nowhere near being put out to pasture. It plays a continuing role in efforts to search out the most distant objects in the Universe, for one thing. Swift is also actively engaged in the study of things closer to hand, including "normal" supernovae. And, Nousek notes, there remains plenty of interest in using it to probe other targets of opportunity. "Last November, on our fifth anniversary, we held a workshop and 250 people came."

In coming years, Swift could be uniquely useful in the ongoing search for gravitational waves, those ripples in spacetime whose existence Einstein predicted in his general theory of relativity back in 1916, and which finally seems on the verge of being confirmed. As Burrows explains, the giant gravitational wave detector known as LIGO, currently undergoing a major upgrade, is expected soon to be sensitive enough to pick up the waves believed to be created by the merging of neutron stars, the same events thought to produce short gamma ray bursts. If LIGO could pick out a gravitational-wave signal from a burst that Swift has identified, it would prove the existence of gravitational waves once and for all—and confirm the merger model to boot.

That kind of joint discovery, says Mészáros, would herald a convergence in observational technologies that seemed far off even five years ago, when Swift was newly launched. And indeed, over that brief stretch of time, as Nousek puts it, "I think it's fair to say that the textbook of gamma-ray burst astronomy has been re-written."

Peter Mészáros, Ph.D., is Eberly Professor of Astronomy and Astrophysics in the Eberly College of Science, and leader of the Swift theory team, John Nousek, Ph.D., is Professor of Astronomy and Astrophysics and Swift's leader for Mission Operations, Derek Fox, Ph.D., is Assistant Professor of Astronomy and Astrophysics and a member of the Swift team, David Burrows, Ph.D., is senior scientist and Professor of Astronomy and Astrophysics and lead scientist for Swift's X-ray Telescope, Peter Roming, Ph.D., formerly senior research associate in Astronomy and Astrophysics and leader of the Swift UV/Optical Telescope, is a staff scientist at the Southwest Research Institute.

Last Updated September 21, 2010