6. GAMMA RAY BURSTS:

Gamma ray bursts were first detected in 1967 by secret US military satellites designed to detect nuclear weapons tests on the Earth. The satellites observed brief flashes of gamma rays. But it wasn't until 1973 that scientists realized that these bursts came, not from the Earth, but from the sky. (See brief history). The fact that the sources radiate mostly gamma rays indicates that they have temperatures of roughly 10⁹ K (a billion degrees) and are among the hottest objects ever observed by astronomers. Until a few years ago, the gamma ray sources had not been detected at any other wavelengths, so astronomers had very few clues as to what the sources were. Indeed, they didn't know whether the bursts were coming from sources in the Milky Way galaxy, at distances of less than 10⁵ light years, or from the most distant reaches of the known universe, at distances of about 10 billion light years.

In 1995 two distinguished astronomers debated the origin of gamma ray bursts. One argued that the gamma ray burst sources were near the Milky Way, and the other argued that they were at cosmological distances. You can read about this debate here. But that debate was inconclusive.

Astronomers observed gamma ray bursts occurring at a rate of roughly one per day with the BATSE detectors on the Compton Gamma Ray Observatory. This map of gamma ray bursts shows that the events are distributed uniformly in the sky, with no concentration toward the Milky Way or any other known astronomical system. Such maps convinced most astronomers that the gamma ray bursts were probably at cosmological distances. But the BATSE detectors could not locate the positions of the gamma ray bursts with sufficient accuracy to find their optical counterparts.

In December 1997, however, Italian astronomers using the Beppo-SAX satellite were able to locate the fading X-ray afterglow of a gamma ray burst with sufficient accuracy that American astronomers could find the source with an optical telescope. A couple of months later, the bright optical source had faded, but the astronomers could see that it was located in a very faint galaxy at a distance of roughly 12 billion light-years. This very important discovery is described here: Gamma-Ray Burst Found to be Most Energetic Event in the Universe. In 1999, astronomers detected an even more luminous gamma ray burst in another distant galaxy: Hubble Views Home Galaxy of Record-Breaking Explosion. That clinched it: most gamma ray bursts are occurring in distant galaxies at the edge of the known universe.

The fact that we now know the distance of gamma ray burst sources is a big step toward understanding what causes them, but we are still a long way from that goal. One of the biggest challenges is to explain how they could be so incredibly bright. If they shine in all directions, these sources must be incredibly luminous -- during the few seconds that they shine, as luminous as 100 million galaxies, each containing 100 billion stars! That would require an energy expenditure equal to that released by 1000 supernovae, and astrophysicists couldn't imagine any system that could release such energy in a minute or less, as gamma ray bursts appear to do.

Of course, the energy required to account for a gamma ray burst goes way down if the gamma ray source doesn't shine in all directions equally, but shines in a narrow beam aimed directly at Earth. For example, if the beam opening angle is only a few degrees, the energy requirement goes down by a factor of 1000 or more, putting the energy required for a gamma ray burst right in the range of the energy release by a supernova explosion.

Of course, if the beaming hypothesis is true, it implies that for every gamma ray burst we observe, there must be 1000 more gamma ray bursts that we fail to see because their beams are not aimed toward Earth. So, although the energy required per source might be reduced by factor 1000, the number of such sources must be increased by the same factor. Astronomers estimate that a typical galaxy in the universe must produce such a burst about once every 100,000 years to account for the number of gamma ray bursts that we see (a few hundred per year).

The figure below illustrates the currently favored model for a gamma ray burst. The energy is released from the source in a narrow jet of particles (probably electrons and positrons) moving at speeds close to the speed of light. The jet travels for a distance of several light minutes (about one Astronomical Unit), emitting a beam of gamma rays as it does. Then, when the jet encounters enough external gas, the impact causes the jet to spread sideways. The gamma ray emission drops rapidly, but an "afterglow" of radio, X-rays, and optical radiation continues to be visible for weeks or months thereafter.

Model for gamma ray burst source (Source: Piran, Science, v. 295, p. 986)

This "relativistic jet model" solves a lot of problems in understanding the observations of gamma ray bursts. The fact that the beam is moving toward us at nearly the speed of light helps to explain why the burst duration is so short, and the idea that the beam strikes external matter provides a natural explanation for the radio, optical and X-ray afterglow that is seen after the gamma rays have faded.

The model makes a interesting prediction: we should be able to see a much greater number of afterglow sources that are not accompanied by a bright gamma ray burst (because the narrow gamma ray beam misses the Earth but the wider afterglow beam does illuminate the Earth). This prediction has yet to be confirmed, but astronomers are now searching for such afterglow sources and they will find them during the next few years if they exist.

The current theoretical model does explain how a beam of relativistic particles having energy comparable to a supernova might produce the bursts of radiation that are observed. But it is an incomplete model. For example, it fails to describe a source of energy that might release this kind of energy in such a narrow beam. One idea is that the burst is caused by the collapse and subsequent explosion of a massive star, and that the beaming might have something to do with the fact that the collapsing star was rotating rapidly. Another idea is that gamma ray bursts are produced when two neutron stars merge, or perhaps when a neutron star is torn apart as it spirals into a black hole, as illustrated by this movie.

If a merger of neutron stars in a binary system causes a gamma ray burst, you should be able to identify one system in the Milky Way that is likely to become a gamma ray burst source, 300 million years from now. (If you can't remember, click here.)

But we still don't have a good theory that can explain how any of these models will produce the observed gamma ray bursts.

The prospects for gaining a better understanding of gamma ray bursts are very good, however. In October 2000, NASA launched the HETE-2 spacecraft, which is designed specifically to catch gamma ray bursts and rapidly to produce fairly accurate locations (within 30 arcseconds) of the X-ray counterparts of the bursts, so that astronomers can study the evolution of the burst and thier afterglows with optical, radio, and X-ray telescopes. HETE-2 has detected and identified more than a dozen bursts in the past year or so.

And now, NASA is building a satellite called SWIFT, intended for launch in September 2003. The gamma ray detectors and X-ray and optical cameras on SWIFT have substantially better angular resolution (2.4 arcseconds) and are more sensitive than those on HETE-2. SWIFT should be able to identify hundreds of gamma ray bursts per year and to provide detailed measurements of the time development of their afterglows.

We hope that the detailed observations of gamma ray bursts with HETE-2 and SWIFT (along with the Hubble Space Telescope and Chandra) will provide the clues that astrophysicists need to develop and test a more complete theory for these mysterious events.

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Last modified March 3, 2002
Copyright by Richard McCray