A Brief History of the Discovery of Cosmic Gamma-Ray Bursts

In October of 1963 the US Air Force launched the first in a series of satellites inspired by a recently signed nuclear test ban treaty. Signatories of this treaty agreed not to test nuclear devices in the atmosphere or in space. These "Vela" (from the Spanish verb velar, to watch) series satellites were part of an unclassified research and development program whose goal was to develop the technology to monitor nuclear tests from space and give the US a means of verifying the conditions of the treaty. The satellites were launched and operated in pairs with two identical satellites on opposite sides of a circular orbit 250,000 kilometers in diameter (about a 4 day orbit) so that no part of the earth was shielded from direct observation. The Vela satellites carried x-ray, gamma-ray, and neutron detectors as a basic instrumentation complement. They also carried a variety of optical and EMP detectors as well as instruments designed to monitor the space environment. The instruments were designed and built by teams of workers at the Los Alamos Scientific Laboratory (now LANL) and Sandia Laboratories of Albuquerque NM, who were specifically assembled and commissioned for the Vela mission.

The x-ray detectors were intended to directly sense the flash of x-rays from a nuclear blast. Although most of the energy of a bomb blast in space would be directly visible as an x-ray flash, a simultaneous indication by the gamma-ray detectors would provide a confirming signature of a nuclear event. A further confirmation would come from the detection of neutrons. The Vela designers were also aware that detonating a nuclear bomb behind a thick shield or on the far side of the moon would effectively hide the initial flash of x-rays from the satellites' view. Hence the gamma-ray detectors could also look for hard gamma-radiation resulting from the cloud of radioactive material blown out after the nuclear blast. This blast cloud could not be totally shielded from view and would expand rapidly. It would easily be detected in gamma-rays even if the detonation took place behind the moon, out of direct view of the satellites' x-ray detectors.

The Vela satellites generally performed well and greatly exceeded their expected operational lifetimes. The satellites' capabilities were steadily improved with each launch. In particular, Vela 5 a and b (launched in 1969) and Vela 6 a and b had sufficient timing accuracy that they could reasonably determine directions to the triggered events. For these later satellites, the light travel time from one spacecraft to another, across the orbital diameter (around 1 second), was greater than the resolution time of the event's onset (about 0.2 seconds). The direction angle to the event with respect to the line between a pair of satellites could thus be determined (to about 1/5th of a radian or 10 degrees) based on the difference in trigger times for the two satellites. Direction angles for a single event observed by multiple pairs of satellites could then be combined to determine one or two possible directions for the source of the event.

In 1965, with the construction and launch of the Vela 3 satellites, Ray Klebesadel of Los Alamos Scientific Laboratory assumed the continuing programatic responsibility for the x-ray and gamma-ray instruments. He saw to it that events which triggered the detectors but were clearly not signatures of nuclear detonations were carefully filed away for future study. In 1972, Ian Strong, also at Los Alamos, was asked to look at Ray Klebesadel's files of Vela gamma-ray events. With the timing accuracy of the later Vela satellites Klebesadel and Strong, along with another Los Alamos colleague, Roy Olsen, were able to deduce the directions to the events with sufficient accuracy to rule out the sun and earth as sources. They concluded that the gamma-ray events were "of cosmic origin". In 1973, this discovery was announced in Ap.J. letters by Klebesadel, Strong, and Olsen. Their paper discusses 16 cosmic gamma-ray bursts observed by Vela 5a,b and Vela 6a,b between July 1969, and July 1972.

Using a hard x-ray detector on board IMP-6 intended to study solar flares, Tom Cline and Upendra Desai of NASA/GSFC were the first to confirm this finding and provide some spectral information that 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 (Wheaton et al. 1973) was also able to confirm a direction to one of the events, supporting the original conclusions 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".