Gamma-Ray Bursts

I. Introduction

Perhaps the greatest mystery for astronomers who look at the sky at very short wavelengths has been the incredibly brief and intense bursts of gamma-rays from seemingly random locations in the sky. A few times a day, the sky lights up with a spectacular flash, or burst, of gamma-rays. Often, this burst outshines all of the other sources of cosmic gamma-rays added together. The source of the burst then disappears completely. No one can predict when the next burst will occur or from what direction in the sky it will come. For thirty years, astronomers have been trying to understand the nature and origin of these gamma-ray bursts (GRBs).

Gamma-ray bursts were first discovered by Ray Klebesadel at Los Alamos National Laboratory. He was working on a project responsible for monitoring the Soviet compliance with the Nuclear Test Ban Treaty. A series of satellites called Vela were put into orbit to perform this task. After the first 4 satellites were up, Klebesadel and his colleagues started to look through the data they sent back to Earth. Primarily, they were looking to make sure everything was working as expected and that nature was not generating any sort of signal that could trick the satellites into thinking a nuclear explosion has occurred. This was a painstaking task of looking through stacks of computer printouts by hand. In fact, instead of graphs that would quickly show what happened, Klebesabel's people had to examine columns of numbers and look for significant changes in their values. In mid-1969, Klebesadel was examining data taken on July 2, 1967. He noticed a spike in the data, a dip, a second spike, and a long, gradual tail off. "One thing that was immediately apparent was that this was not a response to a clandestine nuclear test," Klebesabel said at a conference held about GRBs in Huntsville, AL in 1998. His team checked for possible solar flares and supernovae, and found none.

After this first event was noticed, other similar events were quickly discovered in the data printouts. With the timing between Vela 5 and 6 synchronized to within 1/64th of a second, the Vela team was able to triangulate the locations of the bursts by comparing differences in arrival times at widely separated satellites. They confirmed their suspicion that the bursts came from outside the solar system. Already, by their random scatter across the sky, the data hinted that the sources were out in the Universe rather than being confined in our Galaxy. By 1973, when Klebesadel and his team were ready to publish the results in Nature and present them at the American Astronomical Society meeting, there were at least 16 confirmed bursts.

Using a hard X-ray detector on board the IMP-6 satellite (which was intended to study solar flares), Tom Cline and Upendra Desai of NASA/GSFC were the first to confirm Klebesadel’s findings and provide some spectral information which showed that the burst spectra peaked at gamma-ray energies. Thus the events were not simply the high-energy tail of an X-ray phenomenon. A collimated gamma-ray telescope on board OSO-7 was also able to confirm a direction to one of the events, supporting the original conclusion of cosmic origin. These confirming results, published close on the heels of the original discovery, gave the whole scenario an aura of enhanced mystery. The excitement created in the astronomical community was evidenced by a burst of publications of instrumental and theoretical papers on the newly discovered "cosmic gamma-ray bursts".

Over the next 20 or so years, a catalog of GRBs was constructed and many theories were discussed as to their origin. Great debates were even held within the astronomical community as to whether the bursts were occurring in our Galaxy or in other galaxies. The addition of each newly observed burst tended to reveal not much more than that they never repeated from the same source. The launch of the Compton Gamma-Ray Observatory in 1991 ushered in a new era of GRB observations. The Burst and Transient Source Experiment (BATSE) was capable of monitoring the sky with unprecedented sensitivity. As time passed and the catalog of bursts observed by BATSE grew, one thing became clear: the bursts were in no way correlated with sources in our Galaxy. It began to be accepted that GRBs must originate in galaxies far, far away. In 1997, the Italian-Dutch BeppoSAX satellite made a breakthrough in our understanding of GRBs. Using a particularly effective combination of gamma-ray and X-ray telescopes, BeppoSAX was able to detect afterglows from a few GRBs and precisely locate the sources so that other telescopes could study the same locations in the sky. This work showed that GRBs are indeed produced in very distant galaxies, requiring the explosions producing them to be extremely powerful.

The next big breakthrough in understanding GRBs occurred when an enormously powerful event was detected on January 23, 1999 (designated GRB990123). It was observed with an unprecedented range of wavelengths and timing sensitivities. A small automated optical telescope responded to alerts from orbiting gamma-ray and X-ray telescopes to begin observing the GRB within 22 seconds of the burst’s onset...while the GRB was still on-going. Subsequent observations took place over the next few weeks in the gamma-ray, UV, optical, IR, millimeter, and radio. The object was determined to have a redshift of 1.6, putting it at a cosmological distance and implying a staggering energy release. In fact, if the energy were emitted equally in all directions, twice the rest mass energy of a neutron star would be required. If the energy is being beamed out in a preferred direction that happens in this case to point directly toward Earth, however, the required energies are more reasonable and easier to explain. Multiwavelength, prompt observations of many bursts will be required in order to determine the central engine (or engines...there may be more than one mechanism!) of GRBs.

We tentatively believe GRBs are produced by material shooting towards us at nearly the speed of light, which was ejected during the collision of two neutron stars or black holes. Alternatively, the events could arise from a hypernova, the huge explosion hypothesized to occur when a supermassive star ends its life and collapses into a black hole. However, our sample size is small and our knowledge base shallow.