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

- draft april 17, 1995 - J.Bonnell

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".

(on the First Observed Gamma-Ray Burst ...)

In 1969, while looking back over Vela 4 data just prior to the launch of
Vela 5, Klebesadel and Olsen found an event recorded by Vela 4a,b on July 2,
1967 which also triggered the still operational Vela 3 satellites. The event
appeared to be a cosmic gamma-ray burst but at the time the constellation of
satellites did not have sufficient timing resolution at the trigger to make
a good determination of direction to the burst source. In retrospect, this
event had a time history similar in appearance to the later recognized
cosmic bursts. Klebesadel believes that this event represents the "first
observed gamma-ray burst". The Vela 4 a and b data was used to construct a
time history of this event which was published in Scientific American, Oct.
1976. No earlier observation of a similar event is known.

References:

"Observations of Gamma-Ray Bursts of Cosmic Origin" Klebesadel R.W., Strong
I.B., and Olson R.A. 1973, Ap.J. 182, L85.

"Energy Spectra of Cosmic Gamma-Ray Bursts" Cline, T.L., Desai, U.D.,
Klebesadel, R.W. and Strong, I.B. 1973, Ap.J. 185, L1.

"The Direction and Spectral Variability of a Cosmic Gamma-Ray Burst" Wheaton
W.A., et al. 1973, Ap.J. 185, L57.

"Cosmic Gamma-Ray Bursts" Strong I.B. and Klebesadel R.W. 1976, Sci. Am.
(October Issue, p.66).

"The Vela Satellite Program for Detection of High Altitude Nuclear
Detonations" Singer S., Proceedings of the IEEE 53 December 1965, pp.
1935-1948.

"Systems for the Detection and Identification of Nuclear Explosions in
Space" Dickinson H. and Tamarkin P. Proceedings of the IEEE 53 December
1965, pp. 1921-1934

"Vela Satellites Still Viewing Space", LANL Newsbulletin Nov. 18, 1983, p.
8.

private communications with Ian Strong, Ray Klebesadel, and Tom Cline April,
1995.