That they were the result of a massive star exploding—at least, that was the general belief. The first big advances in understanding gamma-ray bursts came in the 1990s, with the Compton Gamma Ray Observatory, and then a huge advance was made in 1997, with the BeppoSAX Observatory. After 30 years of looking, we finally found the first counterpart of a gamma-ray burst at another wavelength. That was just when the proposal for Swift was being put together to submit to NASA.

“Before Swift was launched, light curves appeared to be described as power laws. Now we know they’re much more complex, and the reason is because Swift has been looking a hundred or a thousand times earlier in the phenomena than we’d ever been capable of looking before.”

Compton looked only with gamma rays. The big thing that BeppoSAX did was find the afterglows of gamma-ray bursts at X-ray wavelengths. But it would take BeppoSAX six to eight hours, often even longer, after a gamma-ray burst went off to find the location and the X-rays. The whole point of Swift was to do that automatically on board, so we could locate the source within a minute or two after the burst went off.

There’s a gamma-ray burst detector on board that localizes the burst to a few arc minutes accuracy on the sky. The space craft is given that position and slews across the sky to that place. It’s a very smart spacecraft and it does this repointing automatically, without any ground intervention. Once it does it, an automatic sequence of observations tries to identify the position of the burst with much higher accuracy. It doesn’t always succeed, but if it doesn’t, the job can usually be done from the ground within a few minutes. Then that position, usually good to about three arc seconds, gets sent out to 800 or 900 astronomers around the world.

Swift can see about two steradians, so about one-sixth of the sky. Typically we see about two bursts per week. In between those, we’ll be following up, doing extended observations on previous bursts, and also looking at other kinds of astronomical objects. Swift, which was launched on November 20, 2004, also has an optical/UV telescope along with the gamma-ray and an X-ray telescope.

That’s right. And I think the reason it’s so highly cited is because anybody who writes a paper about Swift data always cites that article; it’s that article that gives the instrument descriptions.

That was a really key result. There are two types of gamma-ray bursts: they’re generally called long bursts and short bursts. BeppoSAX learned a lot about the long bursts, but very little about the short bursts, because it never triggered on any. So we had very good evidence by the time Swift was launched that the long bursts came from massive stellar explosions. We did not know for sure what caused the short bursts.

It wasn’t until Swift localized a short burst in May 2005 that we were finally able to identify the exact location in the sky for one of them. That burst turned out to be associated with an elliptical galaxy. A couple of months later, two other short bursts were localized: one by the HETE-2 satellite, and the other by Swift. Those two also got very accurate position and optical afterglow observations. So those gave us the first clues about the nature of the short bursts. Several of those came from elliptical galaxies, and that’s critical because elliptical galaxies don’t have massive stars in them. So the kind of progenitor we had for the long bursts could not be causing those short bursts. It had to be something else—and that was the message of the 2005 Nature paper.

If you look at a plot of the frequency distribution of bursts as a function of duration of burst, you see a bimodal distribution. There’s one peak centered around 30 seconds—the ones we call long bursts—and another peak centered around one second—the short bursts. The fact of this bimodal distribution suggests that there are two distinctly different phenomena going on. But the two distributions do overlap, so it’s difficult to tell sometimes with a single burst, to distinguish uniquely which of the two groups it belongs to.

On the long bursts, Swift discovered that the light curves from these were much more complex than we’d ever thought. Before Swift was launched, light curves appeared to be described as power laws. Now we know they’re much more complex, and the reason is because Swift has been looking a hundred or a thousand times earlier in the phenomena than we’d ever been capable of looking before.

We’re seeing a lot of things that we’d never previously seen. We’re seeing these complex light curves; we’re also seeing large X-ray flares, which we think indicates that the initiating process—what we call the central engine driving the burst, which is probably the collapse of a massive star into a black hole—goes on much longer than we previously thought. Instead of that happening in 20 or 30 seconds and being finished, apparently it continues to go on for hours.

Yes.

That’s a subject for debate. You have to try to imagine what could possibly make these gamma-ray bursts and what could be fueling them. One possibility is material falling down into a black hole. If you start with very massive stars, then the standard theory has been that massive stars collapse into black holes. Then the material falling into the black hole powers the gamma-ray burst.

More recent simulations of stellar collapse suggest that it’s also possible for these massive stars to form neutron stars with high magnetic fields, and those could also produce gamma-ray bursts. So we don’t really know for certain which is being produced—a neutron star or a black hole.

Well, we learned that at least some of them, the ones in elliptical galaxies, are not consistent with being formed by massive stars. In those cases, they’re probably formed by the merger of two neutron stars. If two neutron stars are orbiting one another, they will eventually merge and also form a black hole. So we think that’s the mechanism that produces at least some of the short bursts.

We need gravitational wave detections. The observations have all been consistent with neutron star-neutron star mergers or neutron star-black hole mergers. But we don’t have a smoking gun. To get that we need gravitational wave detectors to observe the expected gravitational wave signature of a neutron star merger.

The GLAST satellite will be launched next year and will look at much higher-energy gamma rays than Swift can. The exciting thing about putting up an observatory like GLAST, looking at a new wavelength band or with new sensitivity, is that you always find new things. We know there are some gamma-ray bursts that make extremely high-energy gamma rays, but the data from the Compton observatory was not sufficient to explain what’s going on. GLAST will study those with much more sensitivity.

One of the most exciting things we can expect in the next few years is when Swift and GLAST both observe the same gamma-ray burst. Then we’ll see what it’s doing from optical and UV all the way through X-rays out to these very-high-energy gamma rays. That should tell us a lot.

I was, but I’m not anymore. At the time we were proposing Swift, I was coming in from outside the field. My expertise was detectors. And it seemed like progress on gamma-ray bursts had been very slow. Then the big logjam broke in 1997 when BeppoSAX found the first X-ray afterglow. All of a sudden a huge amount of progress was made and that’s just continued right along.

  Your long-term research goals go beyond gamma-ray bursts. Can you tell us about them?

I’m working on new detector technologies for possible use in future missions like Constellation-X or EDGE. CCDs have been state-of-the-art for X-ray observatories for the past decade, but we are working on developing new detectors that retain the advantages of CCDs—large area and good energy resolution—but improve on them in areas like radiation tolerance and readout rates. We hope that these detectors will be used in the next generation of X-ray astronomy missions, which will feature spectroscopy of X-ray sources using very high throughput mirrors.

The fact that they’re so ephemeral. The burst itself is over in just a few seconds. Even the afterglow that follows from it decays very, very rapidly. Typically the brightness of the X-ray emission decays by a factor of 1,000 or more in the first 20 minutes after the burst. There’s a lot to learn there about everything between us and where the burst is in a very short time. We can study the structure of the intergalactic medium; we can study the structures of high red-shift galaxies, which are galaxies in the very early history of the universe, but we have to look very, very quickly.

David N. Burrows, Ph.D.
Department of Astronomy and Astrophysics
Penn State University
University Park, PA, USA