Before the launch of CGRO, most scientists thought that gamma-ray bursts came from magnetic neutron stars residing in a thick disk in the Milky Way (see, e.g., Higdon and Lingenfelter 1990, Harding 1991). Upper limits to the rate of faint bursts (e.g., Fishman et al. 1979) already implied that the cumulative brightness distribution of gamma-ray bursts must roll over at the faint end. Since a uniform distribution of sources in space requires that the brightness distribution of the bursts follow a power law with -3/2 slope, the rollover meant that gamma-ray bursts must be inhomogeneously distributed in space. Most scientists expected that BATSE would find that the sky distribution of faint bursts is concentrated in the Galactic plane, and would thus confirm that the burst sources lie in a Galactic disk roughly 2 kpc thick.
Instead, the data gathered by BATSE confirmed the existence of a rollover in the cumulative brightness distribution of gamma-ray bursts but showed that the sky distribution of the faint bursts is consistent with isotropy (see Figure 1) (Meegan et al. 1992, Briggs et al. 1995). The rollover in the cumulative brightness distribution and the isotropic sky distribution imply that we are at, or near, the center of the spatial distribution of burst sources and that the intrinsic brightness and/or spatial density of the sources decreases with increasing distance from us. Therefore the bursts cannot come from the Galactic disk.
While an origin for the bursts in the Oort Cloud of comets that exists around the solar system is not ruled out, this model suffers from a lack of any appealing physical mechanism and the likelihood that the Oort Cloud is not highly spherical. Consequently, the primary impact of the BATSE results was to intensify debate about whether the bursts are Galactic or cosmological in origin.
There is no overwhelming piece of evidence, no "smoking gun," which proves that the bursts are Galactic or cosmological. That is why we are having this commemorative debate. As is so often the case at the frontier of science, and as it was in the original "Great Debate," the evidence is circumstantial, even contradictory. Yet in the scientific process, each piece of evidence is weighed. Some pieces are given more weight, others less. And different scientists may give different weights to the same piece of evidence. But eventually, through the process of weighing-up the evidence, scientists reach a conclusion.
Paczynski (1995) focuses on the isotropic sky distribution of gamma-ray bursts. He describes the impact of the announcement that the sky distribution of the faint bursts is consistent with isotropy on him and on some others when it was made by the BATSE team in September 1991 (Meegan et al. 1992). The isotropy of the bursts on the sky is an important piece of evidence. The cosmological hypothesis is consistent with it. But the Galactic hypothesis is also consistent with it (see Figure 2). If this were all of the evidence that we considered, quite frankly we could not distinguish between the cosmological and Galactic hypotheses. I contend that we can go further, by considering other important pieces of evidence, some of them very new. Indeed, since September 1991 many unexpected and important pieces of evidence have been discovered which bear strongly on the question of the distance scale to gamma-ray bursts. Of course the conclusion is not yet clear. But when we consider all of the evidence, I think you will see that it adds up to a strong case for the Galactic hypothesis.
The particular Galactic model that many scientists are now studying is motivated by the discovery that many neutron stars are born with such high velocities that they escape from the Galaxy (see Figure 3). These neutron stars form a distant, previously unknown Galactic "corona". This distant corona contains an ample population of sources which appear isotropic when viewed from Earth, and can therefore easily account for the angular and brightness distributions of the BATSE bursts. The soft gamma-ray burster phenomenon shows that high velocity neutron stars can produce burst-like behavior. Indeed, the famous 1979 March 5 event shows that high velocity neutron stars can produce an event which has the energy, duration, and spectrum of a gamma-ray burst. The Galactic corona model has the attractive feature that it naturally accounts for the many similarities between gamma-ray bursters and soft gamma-ray repeaters, which we know are high velocity neutron stars. In addition, the model easily explains the rapid time variability of many bursts, cyclotron lines, repeating, and the lack of bright optical counterparts.
On the other hand, the cosmological models that many scientists are studying, such as coalescing neutron star binaries and failed supernovae, face severe difficulties in explaining cyclotron lines, repeating, the lack of bright galaxies in the error boxes of bright bursts, and even the energies of the bursts.
Let us now pick up the story of the Galactic hypothesis with the discovery that many neutron stars have very high velocities.