sion of regularity and slow evolution in the They suggested that examination of the Vela exe-atmospheric nuclear detonation. Surpris- universe persisted into the 1960s. data might disclose evidence of bursts of ingly, however, the survey soon revealed that The feeling that transient cosmic events gamma rays at times close to the appearance the gamma-ray instruments on widely sepa- were rare was certainly prevalent in 1959 of supernovae. Such searches were con- rated satellites had sometimes responded when summit meetings were being held be- ducted; however, no distinctive signals were almost identically. Some of these events were tween England, the United States, and found. attributable to solar flare activity. However, Russia to discuss a nuclear test-ban treaty. On the other hand, there was evidence of one particularly distinctive event was dis- One key issue was the ability to detect treaty variability that had been ignored. For exam- covered for which a solar origin seemed violations unambiguously. A leading ple, the earliest x-ray data from small rocket inconsistent. Fortunately, the characteristics proposal for the detection of exo-at- probes and from satellites were often found of this event did not at all resemble those of a mospheric nuclear explosions was the use of to disagree significantly. The quality of the nuclear detonation, and thus the event did satellites with instruments that included de- data, rather than actual variations in the not create concern of a possible test-ban tectors sensitive to the gamma rays emitted sources, was suspected as the reason for treaty violation. by the explosion as well as those emitted these discrepancies. Also, a background of This first tantalizing indication of a cos- later during the radioactive decay of the transient detector responses in much of the mic gamma-ray burst had been found in fission products. During the discussion of the x- and gamma-ray data masked the similar 1969. By 1972, an extension of the search capabilities of these satellites, Stirling Col- responses to true cosmic bursts. These back- had revealed a surprising number of gate (currently a Laboratory scientist, but at ground responses were generated by a events—sixteen bursts over a three-year that time attached to the U.S. State Depart- variety of mechanisms, many due to local period. Each of these bursts, for intervals of ment) suggested that gamma rays from a effects of charged particles trapped in the up to several seconds, dominated the gamma supernova might resemble the radiation of a earth’s magnetosphere and others due to radiation of the entire sky. It was only then nuclear test explosion closely enough to instrumental “glitches” (such as high-voltage that the violent behavior of the cosmos trigger an alarm by the satellite detection arcing, electronic crosstalk, or telemetry er- became clear: chaos and rapid change system. Could this rare event lead to danger- rors). The Vela instruments responded to prevail in the x-ray and gamma-ray regime. ous accusations of a treaty violation? Were these spurious signals frequently enough to there characteristics of a supernova outburst discourage careful inspection of every re- Evolution of Detector Systems that would distinguish between a weapon test cord. However, if the signals were spurious, and a cosmic burst? it would be improbable for more than one Despite the remarkable nature of these In the early 1960s, Los Alamos scientists Vela satellite to have responded at the same events, full awareness of the implications of Jack Asbridge, Sam Bame, Jerry Conner, time. Thus, to identify nonspurious events, the phenomenon developed only gradually. Ray Klebesadel, and Sid Singer, directed by the data were searched for those occurring In fact, other evidence of variability, such as Jim Coon, designed and built detectors for nearly simultaneously between spacecraft. was observed in the quasar 3C273 in 1963, exe-atmospheric nuclear-test surveillance, In However, data records were referenced only was only then causing astronomers to re- the period from 1963 to 1970 six pairs of the to the independent clocks in the spacecraft. consider seriously their view of the universe. Air Force Vela satellites carrying these de- These had to be referred to a common time As the picture of the universe changed, new tectors were placed in orbit far beyond the in order to determine simultaneity. instruments were designed and placed atmosphere (which absorbs gamma rays and Moreover, the detection systems produced aboard satellites to answer a growing list of other nuclear radiations). While these detec- copious numbers of spurious records. Only questions. However, the long lead times and tors served a number of valuable scientific the application of computerized data the space and weight limitations of satellite functions, the initial examination of the processing allowed the search for simul- experiments did not allow rapid action gamma-ray data emphasized the spacecraft’s taneity to be performed on this volume of toward answering these questions. For- primary mission to gather information criti- data. tunately, scientists associated with other cal to U.S. security. Since the concept of a nearly static uni- space projects graciously allowed the piggy- Stirling Colgate and Edward Teller had verse prevailed at this time, it was not backing of unscheduled gamma-ray burst followed up Colgate’s summit meeting com- expected that the search would reveal any- experiments, thus circumventing the usual ment by making specific predictions in 1965 thing extraordinary. The intention was to delay of several years until the next genera- of gamma-ray emission during the initial verify that there were no natural background tion of satellites could be put into orbit. stages of the development of supernovae, events that would mimic the signature of an One of the first questions to be addressed 6 Summer 1982/LOS ALAMOS SCIENCE Vela 5A, 5B, 6A, and 6B 1969, 1970 Geocentric at 1.2 x 10s km Los Alamos Active from Earth International Sun Earth 1978 At a Lagrangian point One instrument by University of California, Berkeley/ Healthy Explorer 3 (ISEE-3) 1.5 x 106 km from Earth Los Alamos and two other instruments by NASA Goddard Space Flight Center Prognoz 7 1978 Geocentric, highly elliptical Centre d’Etude Spatiale des Rayonnements (Toulouse) Telemetry lost in 1980 Einstein Observatory 1978 Geocentric, very near Earth --- Pointing ability (HEAO-2) a lost in 1981 Solar-Maximum Missiona 1979 Geocentric, very near Earth Partially active Helios 2 1976 Heliocentric, highly elliptical NASA Goddard Space Flight Center Telemetry lost in 1980 4 Pioneer Venus Orbiter 1978 Highly elliptical about Venus Los Alamos Healthy Venera 11 and 12 1978 Heliocentric with the two On each satellite one instrument by Centre d’Etude Telemetry lost satellites diverging from Spatiale des Rayonnements (Toulouse) and a second in 1980 each other by A. F. Ioffe Physico-Technical Institute (Leningrad) aA NASA-lauched satellite carrying instruments not specifically designed to observe gamma-ray bursts. The data collected were, however, helpful in analyzing gamma-ray bursts. Several institutions were responsible for the instrumentsaboard. 8 Summer 1982/LOS ALAMOS SCIENCE Cosmic Gamma-Ray Bursts— a continuing mystery straint on the source size because the fluc- tuation time scale cannot be much faster than the travel time of light across a typical source dimension. For a O. 1 -second rise time, a source size less than about 30,000 kilometers is inferred. This is extremely small on an astronomical scale and indicates that compact objects such as white dwarfs, neu- tron stars, or black holes are logical can- didates for the burst sources. of course, if the emission region is very small (a neutron star is only about 10 kilometers across), the characteristic burst time scales probably re- flect other dynamical time scales of the system such as a heating or cooling time. MULTIPLE OUTBURSTS. Another impor- tant time-dependent feature of typical bursts is the complexity of burst waveforms, with Fig. 2. Time dependence of burst intensity. These data, taken on November 4, 1978 multiple outbursts occurring over intervals of with the Los Alamos gamma-ray detection system aboard the Pioneer Venus Orbiter, tens of seconds (Fig. 2). This implies a illustrates the dramatic, highly variable nature of a typical gamma-ray burst. mechanism that is not catastrophic. For example, a supernova explosion would be expected to produce a single outburst. to Siberia and placed on Soviet spacecraft. burst monitoring instruments were in opera- Further, the multiple bursts do not show In addition to the instruments aboard the tion during a burst recorded November 4, strong evidence for periodicities. However, Vela satellites, Los Alamos contributed 1978. This event is the third most intense the burst waveforms often contain similar, toward this network with a modification to burst yet observed, and its features are repeating patterns that suggest systematic the University of California, Berkeley Solar typical of most gamma-ray bursts. Figure 2 and reproducible mechanisms at work. Ad- X-Ray Spectrometer (aboard International shows a record (from Pioneer Venus Orbiter) ditionally. Vela x-ray observations have re- Sun Earth Explorer 3, or ISEE-3) that al- of the burst’s behavior in time. Its intensity cently disclosed repeated outbursts of x rays lowed the instrument to record temporally rises and falls dramatically in fractions of a associated with gamma-ray bursts. The in- resolved spectral data for both gamma-ray second. A number of distinct outbursts, each itial gamma-ray bursts extended to x-ray bursts and solar flares. Also, in a joint lasting on the order of a second, are clearly energies and were followed by additional, development with Sandia National Labor- isolated by periods in which the signal has weaker x-ray outbursts occurring over inter- atories, Los Alamos supplied the gamma-ray returned nearly to the background level.