After three decades of study, gamma-ray bursts still mystify

October 17, 1999

Oct. 15, 1999: For more than three decades, gamma-ray bursts have been one of the most mysterious phenomena in astrophysics. While not as popularly known as black holes, they have stirred public interest as well. Next week, more than 200 of the world's astrophysicists will gather in Huntsville for the 5th biennial Huntsville Gamma Ray Burst Symposium. Science@NASA caught up with Dr. Gerald Fishman (right), the principal investigator on the best burst detector, for an interview about bursts and the symposium.

Science@NASA: Tell us about the 5th Huntsville Gamma Ray Burst Symposium. Who will be attending and what they expect to get out of the symposium?

Fishman: This is the 5th in a series of gamma ray bursts symposium that we have here in Huntsville. Actually researchers from all over the world descend upon Huntsville because Huntsville is the source of the primary data base of gamma ray bursts and has been since the launch of the Compton Gamma Ray Observatory. So it's an appropriate place to hold such a conference.

Science@NASA: What are gamma ray bursts, how were they discovered and how are we studying them?

Fishman: Gamma rays are the highest energy form of radiation. They are higher energy than X-rays; they are very penetrating. They'll go through several inches of steel, for example. So you need very large, massive detectors or instruments in order to stop the gamma rays and to detect them. The Gamma Ray Observatory was launched in 1991 by the Space Shuttle to detect various gamma rays coming from different objects in the universe. These are the highest energy objects that we know of in the universe -- things like black holes, neutron stars, and gamma ray bursts.

The gamma ray bursts were discovered by accident over 30 years ago. No one ever suspected these immense, long flashes of gamma rays coming from different parts of the sky. The United States orbited some nuclear bomb detection satellites to look for clandestine nuclear explosion that might occur in deep space. Of course they didn't find any, but they started seeing little sporadic flashes or bursts of gamma rays at different times. When they started to correlate these among satellites, they saw they were coming in coincidence with each other and that they were coming from different directions in space. They weren't coming from the Earth, they weren't coming from the sun or any other known objects in the solar system. But they were so unsure of themselves, it was such an unexpected discovery, that they waited four or five years until they were really sure they had something new before they announced the discovery in 1973.

Science@NASA: What are the primary instruments that we have for studying bursts now?

Fishman: Since the time after the Vela satellites first discovered gamma ray bursts, there have been different instruments in the 1970s and '80s that detected gamma ray bursts. However it hasn't been until the 1990s that very large dedicated experiments to specifically study the gamma ray bursts have been launched. The foremost among these is the BATSE experiment, the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory that was launched in 1991. Because it has such large detectors and was launched aboard the largest scientific spacecraft ever put into orbit by the U.S., it has a very large sensitivity, high sensitivity to gamma rays. It can detect very faint and therefore very distant gamma ray bursts.

Science@NASA: You mentioned that gamma ray bursts have uncommonly high energies that can penetrate several inches of steel. Obviously conventional telescopes will not work with these. How do we detect them? What is the mechanism?

Fishman: In order to detect gamma rays you need a very special type of what's called telescopes. They need to be heavy and thick and they need to somehow convert the gamma rays into another form of radiation that can be more easily detected. For this we use scintillation crystal detectors. A scintillation crystal has a property that when radiation hits it, it gives off a little flash of light or what we call scintillation. These faint flashes of light are then detected by other optical detectors. Then the pulses are transmitted to the ground.

Science@NASA: How many bursts have we observed with BATSE and what have we learned from the eight-plus years of observation with it?

Fishman: Initially gamma ray bursts, because they were studied with only small detectors, were only seen about 10 to 20 times per year. Since the launch of the BATSE experiment we've been seeing about one a day or 300 per year. We now have a catalog of over 2,500 gamma ray bursts which is over half of the gamma ray bursts ever seen since their discovery. We've been able to characterize the various properties of these gamma ray bursts -- that is, their distribution over the sky, their intensity distribution, how the different energies of gamma rays are spewed forth.

From these observations come many theories -- in fact, there's over a 150 theories -- of what causes a gamma ray burst. People are starting to settle in on a couple of specific ideas. It wasn't until after the BATSE experiment was launched that we realized that gamma ray bursts were at the distant edges of the universe. Before that most people thought that they were relatively nearby in our own galaxy. Because they are so distant, they have a tremendous of energy. In fact, they are the largest known explosions in the universe, thousands of times brighter than supernovae. But in recent years, it has occurred to several people that supernovae and gamma ray bursts may be somehow related. Because we know that both objects come from a compact small region and they somehow involve explosions.

Many people think that this is due to the collapse of a super massive star or perhaps the combination or merger of two very dense compact objects such as neutron stars or a neutron star and a black hole. This would lead to a tremendous outpouring of energy. For example, if you had a marshmallow fall into a neutron star, it would produce as much energy as a thousand hydrogen bombs. Well we are talking about here a significant part of a star falling onto a neutron star. This would cause enormous explosion. In fact the amount of energy that they put out is more energy than the sun puts out in its entire 10 billion-year lifetime.

Science@NASA: From what we know most of these are very distant in the universe and therefore very old in time. Is this a phenomenon that we are likely to see happen in our galaxy or has already happen in the distant past?

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Fishman: The best estimate of how often a gamma ray bursts occur is about once every hundred million years. Since our own galaxy is probably 10 or 15 billion years old, yes, many of them have happened in our own galaxy. In fact, some people think that perhaps that was the reason the dinosaurs became extinct maybe 65 million years ago, because a nearby gamma ray bursts disturbed the Earth's atmosphere and caused the dinosaurs to die.

Science@NASA: But there has also been some thinking that perhaps the shocks waves from gamma ray bursts could even play a role in planetary formation?

Fishman: A recent paper has speculated that perhaps a very large explosion is what compresses the interstellar medium to actually form planets. It is a very speculative thought but a interesting one. Because others that have tried to look into planetary formation have had trouble condensing the very diffuse interstellar matter and it could it be that just such an explosion may help condense interstellar material and form new planets.

Science@NASA: What other objects can BATSE study?

Fishman: Gamma rays are given off by the most exotic and energetic objects in the universe and BATSE can study these as well as gamma ray bursts. Because we have eight BATSE detectors and because we look at the entire sky, we're sensitive to any transient or explosive object or a large flare or flickering from known objects. Examples of things that we've seen are black holes within our own galaxy, neutron stars, binary neutron star systems, pulsars, solar flares and very interesting objects known as microquasars, which are believed to be black holes within our galaxy that spew out very narrow, collimated jets, very fine jets of material close to the speed of light. These produce gamma rays as well.

Science@NASA: But among these are the most intriguing - microquasars sound neat - but the most intriguing, certainly the most newsworthy over the last year and half are the magnetars.

Fishman: It's been known for some time that neutron stars have a very high magnetic fields. But it is only recently, the past year and a half, that it has become recognized that there is a certain class of neutron stars we call magnetars that have an extraordinary high magnetic field. This magnetic field is so strong that when a perturbation or a what's called a starquake in a neutron star occurs, this intense magnetic field is shaken and it accelerates particles to very high energies and we see this as an enormous blast of x-rays and gamma rays.

But there are only four magnetars that are known now. Three of them are in our own galaxy and one of them is in a neighboring galaxy, the Large Magellanic Cloud. The magnetar theory is very interesting because it predicts types of radiation and phenomena that we can't possibly study here on Earth because of the magnetic fields of a magnetar are billions times stronger than anything we can create in a laboratory.

Science@NASA: And one last surprise that BATSE gave us apparently comes from thunderstorms?

Fishman: Yes, this was a complete surprise. Probably the last place we ever expected to see gamma rays coming from is the Earth itself. But soon after launch, about once a month, we would see a very brief flash, an intense flash of gamma rays, lasting only a few thousandths of a second. When we analyzed the data we found they we coming not only from the Earth but near large thunderstorm systems on the Earth. This was completely unexpected.

At the same time other people have been studying what are called sprites. This is some sort of electrical discharge phenomena that occurs between the tops of thunderstorms and go upwards all the way through the stratosphere to the ionosphere. It is thought that the same conditions that are capable of producing these beautiful red optical sprites are also capable of producing gamma rays. However it is still speculative.

Science@NASA: And finally what is the next step beyond Compton and beyond BATSE?

Fishman: Well the real key to making progress in gamma ray bursts is to identify where the bursts occur very precisely so you can look at the host galaxy. That is what kind of a galaxy it was formed in, and what the environment of the gamma ray burst source is like. A breakthrough came about three years ago from the Italian-Dutch satellite Beppo SAX. For the first time, they were able to precisely and quickly get the direction to a gamma ray burst so that x-ray and optical follow-up observations could be made. Ever since gamma ray bursts were discovered, people thought what we really needed was to find the counterpart, that is to study a gamma ray burst in some other part of the electromagnetic spectrum.

BATSE can't locate the gamma ray bursts very precisely so it was up to this Italian-Dutch satellite, Beppo SAX, to do that job. They discovered that, sure enough, there was x-ray and optical radiation that lasted for several days or even weeks after the initial explosion. The Hubble Space Telescope has found very faint but positive afterglow emission coming from some of these gamma ray bursts locations as much as six months after the initial explosion.

This has led to a revolution in the development of models as to what's causing the gamma ray bursts and how the radiation comes out as what is called a fireball, a relativistic blast wave of radiation that slams into the nearby interstellar medium. Because gamma ray bursts are so far away we can actually use gamma ray bursts to study the most distant and therefore the earliest regions of the universe. We expect they are going to become as useful for these early universe studies in cosmologies as supernova have become. So the push in the future is to get more satellites up there that can precisely locate gamma ray bursts and provide this information to astronomers that use ground-based and space-based telescopes.

The next such mission will be launched in January. It is called the HEDI II mission, led by a group of scientists at MIT. Following that we expect to hear very shortly that a new mission has been approved called SWIFT. SWIFT has a variety of telescopes on board as well as gamma ray detectors that will be able to quickly respond to gamma ray bursts. And then perhaps ten years from now we expect a major facility called the Next Generation Gamma-Ray Burst Observatory to provide us with perhaps thousands of very precise gamma ray burst observations with good locations.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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