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Between the Compton Gamma Ray Observatory (CGRO) launch in 1991 April and the Pioneer Venus Orbiter (PVO) demise in 1992 October, concurrent coverage by CGRO, PVO, and Ulysses was obtained for several hundred gamma-ray bursts (GRBs). Although most of these were below the PVO and Ulysses thresholds, 37 were positively detected by all three spacecraft, with data quality adequate for quantitative localization analysis. All were localized independently to ~2° accuracy by the CGRO Burst and Transient Source Experiment (BATSE), and three were also localized by COMPTEL. We computed arrival-time error boxes, whose larger dimensions range from about 2' to several degrees and whose smaller dimensions are in the arcminute range. Twelve have areas less than 10 arcmin2, and only four have areas greater than 1 deg2. The area of the smallest box is 0.44 arcmin2. We find that the overall BATSE localization accuracy for these events is consistent with the most recent stated uncertainties. This work indicates that the ROSAT soft X-ray source found within a preliminary IPN error box for GB920501 (Trig 1576) (Hurley et al.) is less likely to be the GRB counterpart than previously reported.

The high precision gamma-ray spectrometer (PGS) is scheduled to be launched on the Russian MARS mission in 1996, and to go into an elliptical polar orbit around Mars. The PGS consists of two high-purity germanium detectors, associated electronics, and a passive cooler and will be deployed from one of the solar panels. The PGS will measure nuclear gamma-ray emissions from the Martian surface, cosmic gamma-ray bursts, and the high-energy component of solar flares in the broad energy range from 50 keV to 8 MeV in 4096 energy channels. The first results are presented of development, integration and qualification of the instrument, both for the passive cooler and for the detector with spectrometric electronics.

Long-term (1979–88) observations of relativistic electrons in the earth's outer magnetosphere show a strong solar cycle dependence with a prominent intensity maximum during the approach to solar minimum (1983–85). This population therefore corresponds to the presence of high-speed solar wind streams emanating from solar coronal holes. Using a numerical code, we have calculated the precipitating electron energy deposition in the earth's upper and middle atmosphere. Observed events (typically persisting several days) would have maximum effect in the 40–60 km altitude range. We suggest that this electron population could play an important long-term role in modulating lower D-region ionization and middle atmospheric ozone chemistry. We describe methods of observing middle atmospheric and lower ionospheric effects of the electrons. A particularly promising approach may involve the monitoring of global Schumann resonance modes which are sensitive to global changes in the properties of the earth-ionosphere cavity. Ongoing work indicates that Schumann resonance properties are moderately correlated with the flux of precipitating relativistic electrons thus offering the possibility of continuously monitoring this aspect of magnetosphere-atmosphere coupling.

THE Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory has shown that, although γ-ray bursts are distributed isotropically on the sky, there is an apparent dearth of weak events compared to those expected from a homogeneous distribution of sources1,2. This suggests that the bursts originate either in an extended galactic halo3 (that is, locally) or at cosmological distances4,5. The intensity distribution of the bursts can be used to constrain their source properties and spatial distribution, and here we address this question by considering the BATSE data with those from the Pioneer Venus Orbiter, correcting for the different reponse of the two instruments. We show that the composite intensity distribution can after all be fitted by a simple, homogeneous model of γ-ray burst sources with identical intrinsic luminosities where the faintest BATSE events are at a redshift of z∼0.80±0.05. We obtain an upper limit of about −18 on the absolute magnitude of the host galaxies by assigning model-derived distances to the brightest extragalactic objects found15 within the error boxes of eight well localized γ-ray bursts. All but the faintest active galaxies are excluded as the source of the bursts. The bursts may instead be associated with normal galaxies, but only if the host-galaxy magnitudes are close to our upper limit.

Gamma-ray burst detectors are aboard seven U.S. Air Force Defense Meteorological Satellite Program (DMSP) spacecraft, two of which are currently in use. Their 800 km altitude orbits give a field of view to 117 degrees from the zenith. A great many bursts have been detected, usually in coincidence with detections by GRO or other satellites such as PVO or Ulysses. The directions of the sources can be determined with considerable accuracy from such correlated observations, even when neither GRO nor BeppoSAX is involved. Results obtained from the most recently launched satellites (DMSP 13 and DMSP 14) are given in this paper.

Gamma-ray burst detectors are aboard six U.S. Air Force Defense Meteorological Satellite Program (DMSP) spacecraft, two of which are currently in use. Their 800-km altitude orbits give a field of view to 117° from the zenith. A great many bursts have been detected, usually in coincidence with detections by GRO or other satellites such as PVO or ULYSSES. The directions of the sources can be determined with considerable accuracy from such correlated observations, even when GRO/BATSE with its directional capabilities is not involved. Thus these DMSP data, especially in conjunction with other observations, should be helpful in trying to understand the true nature of gamma-ray bursts.

The discovery of cosmic gamma-ray bursts, based on an analysis of data produced by the Vela defense satellites was announced in Ap.J. letters in 1973 by Klebesadel, Strong, and Olsen. The story of the discovery begins 10 years earlier, in October of 1963, when the US Air Force launched the first in this series of satellites inspired by a nuclear test ban treaty. Interest in the events surrounding this discovery has been piqued by the recent public debate on the distance scale to the burst sources. Details of the Vela program to detect nuclear detonations and general circumstances surrounding the discovery of gamma-ray bursts of “cosmic origin” are presented.

The interplanetary gamma‐ray burst network of the Ulysses, Compton‐GRO, and Pioneer‐Venus Orbiter missions has made source localizations with fractional‐arc‐minute precision for a number of events, and with auxiliary data, will provide useful annular‐segment loci for many more. These studies have, thus far, yielded one possible counterpart, a Rosat x‐ray association with the 92 May 1 burst. Similar to the historic 1978 November 19 burst/Einstein association, this possibility gives hope that network studies will provide a fundamental source clue for ‘classical’ bursts, just as a second supernova remnant in a network‐defined source field has done for sgr events.

Gamma‐ray burst detectors are aboard three U. S. Air Force Defense Meteorological Satellite Program (DMSP) spacecraft, in orbit at 800 km altitude, with corresponding fields of view to 117° from the zenith. A large number of bursts have been detected by DMSP, usually confirming and supplementing data from GRO and other spacecraft. The location of a gamma‐ray burst source detected by several DMSP spacecraft is considerably restricted by knowledge of the several fields of view. Often non‐detection of a strong burst by one or more DMSP spacecraft is even more informative in narrowing the possible area of the burst. The DMSP data in conjunction with observations by other spacecraft can lead to reasonable positional information when more accurate positions are not from GRO or other data.

Using the ROSAT all‐sky survey data, we examined recent small GRB error boxes. In one case, a source has been found inside the GRB error box of size 13 arcmin2. Upper limits to the source count rates are derived and discussed.

The WATCH all sky monitors aboard the Granat and EURECA spacecraft have the capability of independently localizing gamma‐ray bursts to error circles whose 3 sigma radii are 1 degree or less. These are the most accurate single‐experiment localizations currently achievable. In those cases where both WATCH and one or more experiments from the IPN detect a burst, the localizations may be refined considerably. We have identified approximately 35 bursts between 1991 and 1993 in this category. Some were detected by WATCH, Ulysses, PVO, and BATSE, and so on. We present and compare the locations of some of these bursts.

We report the detection of a soft X‐ray source in the error box of GBS J0815−3245=GRB 920501, during a TOO observation by the ROSAT X‐ray satellite. Some preliminary results of this observation are presented.

The third and latest interplanetary network for the precise directional analysis of gamma ray bursts consists of the Burst and Transient Source Experiment in Compton Gamma Ray Observatory and instruments on Pioneer‐Venus Orbiter and the deep‐space mission Ulysses. The unsurpassed resolution of the BATSE instrument, the use of refined analysis techniques, and Ulysses’ distance of up to 6 AU all contribute to a potential for greater precision than had been achieved with former networks. Also, the departure of Ulysses from the ecliptic plane in 1992 avoids any positional alignment of the three instruments that would lessen the source directional accuracy.The 1.5‐year life of this network has now been adequate to provide at least precisely determined source fields and 30 narrow, elongated source loci. The results as of the time of writing, like the results (other than N49) of former networks, show no positional agreement with any previously catalogued candidate objects. An exposure of a preliminary source fie...

The long duration of the Pioneer Venus Orbitor (PVO) mission has provided coverage of the infrequent, intense Gamma‐Ray Bursts (GRB) that occupy the −3/2 power law region of the log N‐log P distribution. The combination of the PVO events with the BATSE results on weaker events could provide the frequency distribution of GRBs over a range of 104 in intensity. Earlier work combined the BATSE and PVO results and concluded that they are not consistent with simple (i.e., no evolution) cosmological models involving standard candles. However, to utilize both databases, careful attention must be paid to the normalization of each data set. This paper presents a new renormalization of the PVO log N‐log P distribution which, when compared with the BATSE results, reduces the disagreement with the simple cosmological models from a greater than 3σ result to one between 2 and 3σ.

Gamma‐ray burst detectors are aboard three U.S. Air Force Defense Meteorological Satellite Program (DMSP) spacecraft, in orbit at 800 km altitude. A large number of burst have been detected by DMSP, often confirming and supplementing data from GRO and other spacecraft, sometimes detecting bursts not othewise known. The position of an unknown source may be considerably restricted by knowledge of the several DMSP spacecraft locations and fields of view. These data may be of considerable assistance in understanding the gamma‐ray burst phenomenon.

A nuclear fission debris tracker, generally referred to as Package B, has been flown aboard Defense Meteorological Satellite Program (DMSP) satellites since 1976. More recently, this instrument has been redesigned in order to obtain additional functions. Since June 1987, four redesigned instruments have been flown. The first portion of this report presents a detailed description of the recent instruments and orbital parameters. The second half of the report concentrates on data representing periods of background and events of scientific interest.

The Pioneer Venus Orbiter (PVO) log N‐log P distribution follows a −3/2 power law for over a factor of 30 in intensity, which is inconsistent with Gamma‐Ray Bursts from large redshifts. We develop log N‐log P models suitable for cosmological gamma‐ray bursts. The PVO data is best fit with a standard candle peak luminosity of 2×1050 erg/sec with 3×104 events/yr within a Hubble distance. However, non‐cosmological models fit nearly as well (L0≪1050). A luminosity of 1052 erg/sec fits at a 3σ level and is, therefore, a maximum upper limit allowed by the PVO data. This upper limit is roughly equal to other reported lower limits. The BATSE event rate combined with the PVO limits imply that evolution with epoch must be occurring.

The new interplanetary gamma‐ray burst network will determine source fields with unprecedented accuracy. The baseline of the Ulysses mission and the locations of Pioneer‐Venus Orbiter and of Mars Observer will ensure precision to a few tens of arc seconds. Combined with the event phenomenologies of the Burst and Transient Source Experiment on Compton Observatory, the source locations to be achieved with this network may provide a basic new understanding of the puzzle of gamma ray bursts.

A number of gamma-ray bursts have been detected by means of gamma-ray detectors aboard three Air Force Defense Meteorological Satellite Program (DMSP) satellites, in polar orbits at 800 km altitude. The gamma-ray data have a 2-second resolving time, and are usually telemetered in 5 energy bins in the range 50--1000 keV. Although it is not possible to detect gamma-ray bursts when the DMSP satellites are passing through the radiation belt or the South Atlantic Anomaly, or when the source is obscured by the Earth, a number of gamma-ray bursts have been detected by two or even three of the satellites. The DMSP data may be of considerable, assistance in evaluating time histories, locations, and spectra of gamma-ray bursts.

We recently discovered 217 'new' celestial gamma-ray burst candidates from the 'new' burst search of the PVO real time data base. The burst search covered the time period from September 1978 to July 1988. Sixty were confirmed by at least one other spacecraft, e.g., ISEE-3, V-II, V-12, etc. None triggered the PVO high time resolution memory. In this paper we describe a new algorithm based on the relationship between time width T(w) and hardness ratio HR, to distinguish cosmic gamma-ray bursts from solar flares without knowing the directions of the events. The algorithm was tested with 83 triggered and 60 nontriggered confirmed gamma-ray bursts and 30 confirmed solar flares from PVO.

RESULTS from the Burst and Transient Source Experiment (BATSE)1 on the Compton Gamma-Ray Observatory show a relative deficiency of faint γ-ray bursts without any detectable anisotropy. This gives strong support to suggestions that the bursts originate at cosmological distances2, such that cosmic expansion reduces the detectability of faint (and therefore distant) sources relative to a uniformly filled euclidean space. But the Pioneer Venus Orbiter (PVO) observations of brighter bursts show no such deficiency, indicating that the bursts that it sees are at redshifts less than about one. The best-fit cosmological model based on PVO data (peak luminosity L0 = 2 x 1050erg s-1) with no evolution of the source population predicts, when extrapolated to the fainter sources, a factor of 40 more events than BATSE sees. Even the 3a limit from PVO (L0 = 9.1 x 1051 ergs-1) is barely consistent with the BATSE result. Evolution of the cosmological γ-ray burst sources with epoch therefore seems necessary if the PVO and BATSE data are to be reconciled. The required evolutionary trend is for fewer or fainter bursts at larger redshift.

The March 5th, 1979 gamma-ray transient has long been thought to be fundamentally different from the classic gamma-ray bursts (GRBs). It had recurrences, pulsations, and a soft spectral component unlike classic GRBs. With the exception of the soft component reported from the Konus experiment, the unusual characteristics of March 5th were detectable main peak differs markedly from the published Konus spectrum. Rather than being dominated by a soft component similar to that observed in the soft gamma repeaters (SGRs), the ICE-PVO spectrum appears to be consistent with a classic GRB spectrum, especially above 100 keV. We believe that, given the ICE-PVO spectral observations, the March 5th transient would have been classified as a classic GRB when it was discovered. The SGRs and GRBs could be consanguineous: high-velocity neutron stars initially produce SGR events (and, occasionally a GRB like March 5th) and when they are older and in the galactic corona, they go through a GRB phase. The March 5th event demonstrates that high-velocity neutron stars at distances of tens kpc are capable of producing events like classic GRBs., 32 pages, uuencoded Postscript with figures. Also available via ftp at ftp://nis-ftp.lanl.gov/pub/grb/march5.ps

A gamma-ray burst which occurred on 1992 May 1 was observed by three spacecraft in the third interplanetary network, and rapidly localized to a small error box. The coordinates were promptly circulated to a wide astronomical community, and radio, optical, and X-ray counterpart searches were carried out. A weak X-ray source was found in the error box, and two radio sources are discovered outside the error box, but in alignment with the X-ray source. The X-ray source position contains approximately 25 optical objects down to 23d magnitude. We discuss the prospects for identifying the burster counterpart.

The study of soft gamma repeaters began with the detection of the soft gamma-ray burst of 1979 March 5 that was followed by long-lasting 8 s pulsations. Later weaker bursts were detected from the same source, although they did not exhibit detectable pulsations. Here we analyze data from the UCB/Los Alamos experiment on the International Cometary Explorer (ICE) of a different soft gamma repeater, SGR 1806-20. A search for periodicities in data following the bursts showed a signal at 2.8 s. The probability that a periodic signal of this size would occur by chance somewhere in the power spectrum is ∼1.3%. The detected amplitude is ∼25 times weaker than what would result if the SGR 1806-20 bursts were simply scaled-down replicas of the March 5th event

The homogeneity of the burst source population in space with the log N(greater than P sub c)-log P sub c and log (dN/dP sub c)-log P sub c relations is studied on the basis of a sample of 225 observed gamma-ray bursts between September 1978 and July 1988 from the Pioneer Venus Orbiter. The relations N(greater than P sub c) is approximately equal to P sub c exp -1.5 and dN/dP sub c is approximately equal to P sub c exp -2.5 are expected if the gamma bursters are uniformly distributed in space. The Kolmogorov-Smirnov statistic gives a probability of 0.93 that the observed integral distribution N(greater than P sub c) is drawn from the expected uniform distribution P sub c exp -1.5. A least-squares 1 standard deviation fit to the observed differential distribution yields dN/dP sub c of about P sub c -2.42 +/- 0.07. Both are consistent with a uniform distribution in space of the parent population of burst sources.

The vertical structure of the impulsive and gradual hard X-ray sources in high coronae and the characteristics of the impulsive soft X-ray emission are investigated on the basis of PVE, ICE, and GOES observations of the energetic flare on February 16, 1984. The average photon spectra observed by these instruments during the impulsive and gradual hard X-ray bursts are summarized. A comparison of these unocculted and partially occulted spectra shows that the sources of the impulsive hard X-ray (greater than about 25 keV) and impulsive soft X-ray (2-5 keV) emissions in this flare extended to coronal altitudes greater than about 200,000 km above the photosphere. At about 100 keV, the ratio of the coronal source brightness to the total source brightness was 0.001 during the impulsive phase and less than about 0.01 during the gradual hard X-ray burst. The sources of the gradual hard X-ray burst and gradual soft X-ray burst were almost completely occulted, indicating that these sources were located at heights less than 200,000 km above the photosphere.

Using electron count rate data at geostationary orbit, daily energy spectra, extending from 30 keV to 15 MeV, have been developed for trapped relativistic electrons at 6.6 earth radii. These spectra have been used to model the flux of these electrons into the atmosphere at 120 km. Energy deposition calculations permit daily sources of HO(x) and NO(y) to be calculated at auroral and subauroral latitudes due to relativistic electron precipitation (REP) for the period June 13, 1979, through June 4, 1988. Both short-term and long-term source variations are quite large over the period considered. The results suggest that a significant contribution to the anomalously large and unexplained global O3 declines between 1979 and 1985 has been made by the catalytic destruction of O3 by odd nitrogen in the lower stratosphere at mid to high latitudes. The results also provide evidence for a clear and strong linkage between solar variability, the state of the magnetosphere, and the chemical climatological state of the middle and lower atmosphere.

The relationship between relativistic electron flux variations at 6.6 R(E) and various published indices and solar wind properties is studied. An attempt is made to characterize the temporal and dimensional relationships between solar wind or magnetic indices and electron flux enhancements. It is shown that the solar wind speed upstream of the earth's magnetosphere is correlated with large increases in the flux of highly relativistic electrons at geostationary orbit.

The results of a search for the soft and short Gamma-Ray Bursts (GRBs) in the data of the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) are presented. Data for four events are presented, including their time profiles and spectral characteristics. In one case the instrument time resolution reveals a total burst duration of 55 ms with rise and decay times of less than about 5 ms.

We have discovered a correlation between the Venera spacecraft locations and the gamma-ray burst positions reported in the KONUS catalog (Mazetset al., 1981a). The reason for the correlation is not clear, but it could be due to spatial selection effects and/or large localization errors for weak of soft bursts. Whatever the cause, it seems likely that this systematic bias might significantly affect catalog results pertaining to gamma burst locations, intensity distributions, and spectra. For example, it can explain why the KONUS galactic latitude distribution is peaked significantly south of the galactic equator. Apparent discrepancies between the KONUS and Los Alamos gamma burst data bases are noted.

Twenty-seven-day periodic enhancements of the relativistic electrons are observed in association with concurrently measured solar wind streams, and a numerical transport code is used to investigate the coupling of these high-energy electrons to earth's upper and middle atmosphere. When precipitated, these electrons are found to show a large energy deposition at 40-60 km altitude which is 3-4 orders of magnitude greater than the galactic cosmic ray or solar EUV energy deposition at these altitudes. It is suggested that this electron population could play a role in coupling solar wind and magnetospheric variability to the middle atmosphere through a modulating effect on lower D-region ionization and possibly on upper level ozone chemistry.

We describe the recent discovery by the Vela satellite system of short, extremely intense, bursts of gamma rays originating outside the solar system, and the subsequent confirmation by a number of other independent observers. The known properties of these so-called gamma-ray bursts will be given together with outlines of some of the more promising explanations produced so far. Essentially all these explanations invoke compact stellar objects - that is, dwarf stars, neutron stars or black holes.

The energetic photon spectrometers aboard the ICE and PVO (Pioneer Venus Orbiter) are described briefly, and the procedure for their in-flight calibrations is discussed. Successful intercalibration of these two instruments led to stereoscopic observations of 100 keV-2 MeV photon sources in solar flares and the study of the directivity and height structure of these sources. The impulsive hard X-ray source is found to extend from the chromosphere to the corona, the brightness of the source decreasing rapidly with increase in height above the chromosphere. The analysis so far indicates no systematic directivity for the hard X-ray source. The observations are consistent with energetic electrons accelerated in the corona propagating downward toward the chromosphere. However, when avaraged over the duration of an impulsive hard X-ray flare, the 'beaming' of electrons is found to be small in most flares.

Many spacecraft operational problems in the earth's outer magnetosphere appear to be due to intense, transient radiation phenomena. Three types of naturally occuring, and highly variable, hostile particle radiation environments are encountered at, or near, the geostationary orbit: (1) high-energy protons due to solar flares; (2) energetic ions and electrons produced by magnetospheric substorms; and (3) very-high-energy electrons of uncertain origin. In this paper, particular emphasis is given to highly relativistic electrons (3–10 MeB). Electron fluxes and energy spectra are shown which were measured by two high-energy electron sensory systems at 6.6 R E from 1979 through 1984. Large, persistent increases in this population were found to be relatively infrequent and sporadic in 1978-81 around solar maximum. During the approach to solar minimum (1981-present) it is observed that the highly relastivistic electrons occur with occur with a regular 27-day periodicity, and are well associated with the re-established solar wind stream structures. Through a superposed epoch analysis technique we show that an energetic electron enhancement typically rises on 2- to 3-day time scale and decays on 3- to 4-day scale at essentially all energies above ca. 3 MeV. The present analysis suggests that these electrons have a very deletrious influence on spacecraft systems due to deep dielectric charging and low-dose susceptibility effects.

The preliminary results from optical search for light pulses associated with gamma ray bursts by means of the Czechoslovak Fireball Network plate collection at the Ondřejov observatory are given. Optical monitoring represents more than 7700 hours, but no real optical counterpart was found. Problems associated with the optical search for gamma ray bursts are discussed.

Spectral measurements by the Solar Maximum Mission have been used to confirm the cyclotron lines in gamma-ray bursts reported from the Konus experiment. We present ISEE-3 data for the same burst (GB800419) during the same period of time which show no line. We discuss various problems in the analysis of scintillator spectra and point out that unfolded spectra are not necessarily unique and that the position of a data point in a deconvolved spectrum may vary depending on the assumed overall shape of the spectrum. As a result, if the analysis assumes a soft spectrum (such as optically thin thermal bremsstrahlung) an absorption feature might appear, whereas a harder spectrum (such as a Comptonized blackbody) would not require the feature. Since the continuum shape probably changes during the duration of a typical burst, the nonuniqueness of the spectral unfolding, combined with the assumption that the continuum is optically thin thermal bremsstrahlung, could give rise to spurious “absorption” features which vary on a time scale of seconds. Despite these problems, there is still some evidence for narrow spectral lines in the range 45 to 65 keV but not for the broad lines reported from the Konus experiment. Unfortunately, the range 45 to 65 keV is the most difficult spectral region to unfold.

Observational data on gamma-ray bursts are reviewed. Information is grouped into temporal properties, energy fluxes and spectral properties, and directions and distributions of the sources in space. (BJG)

EXOSAT observations of the 19 Nov 1978 and 25 Mar 1979b gamma bursters are presented, for the 0.02–2.5 keV energy range. No source was detected in either case, leading to temperature upper limits for the neutron stars assumed to be present of 10 5 –10 6 °K. Polar cap accretion rates are constrained to values of 10 −12 –10 −18 M ⊙ /y km 2 . The EXOSAT non-detection of the source discovered by the Einstein satellite in the 19 Nov 1978 error box may be explained if the burster is more distant than about 1.5 kpc.

Preliminary results of an investigation of the Ondrejov and Sonneberg plate collections for possible optical counterparts to gamma ray bursters (GRBs) are presented. The study proceeds in three directions: as time-correlated searches for GRBs with known positions, as time-correlated searches for GRBs without known positions, and as archival searches. The position of the Perseus flasher was also investigated using 1000 hours of monitoring. No evidence for true optical images associated with GRBs was found. Statistical conclusions are drawn and discussed.

Gamma-ray burst observations performed by LASL began with the identification and initial report of the phenomenon from data acquired by the Vela satellites. The Vela instruments have recorded responses to 73 gamma-ray bursts over a ten-year interval, and are continuing to contribute toward these observations. Similar instrumentation was included aboard the NRL SOLRAD 11 spacecraft. These performed well but suffered an early demise. Recently, the LASL gamma-ray burst astronomy program has been enhanced through the implementation of experiments aboard the Pioneer Venus Orbiter and ISEE-C spacecraft. Both of these experiments are continuing to contribute data vital to trigonometric directional analyses.

The error box of the unusual Gamma-Ray Burst of March 5, 1979 falls completely inside the optical and radio image of the Supernova Remnant N49 in the Large Magellanic Cloud. This region was observed twice in x-rays with the High Resolution Imager of the Einstein Observatory, six weeks and nearly two years after the Gamma-Ray Burst. We show the comparison between the two observations.

The occasional occurrence of brief but intense bursts of cosmic gamma-rays was disclosed through a systematic search of data acquired from the Vela satellites. Confirmation of the nature of the events and additional detail of their characteristics has subsequently been provided by 15 other groups of experimenters with instruments on 13 spacecraft. Thirty-nine such events have been identified from data spanning a period of four and a half years. The record of intensity as a function of time varies considerably for different events, with total durations ranging from 0.1 to ∼ 60s. Time-integrated flux density ranges from ∼10−6 to 10−3 erg cm−2. Spectral measurements have been accomplished by several groups of experimenters, showing a broad maximum in the energy distribution at about 150 keV. The distribution of source directions implies either near galactic or extragalactic locations. The existing data are not sufficient to distinguish between the various models proposed to explain the phenomenon; no model is completely consistent with all observed characteristics.

The Vela-4 satellite energetic-particle experiment consists of (a) a pair of diffused-junction semiconductor detectors for electrons in the energy range 30-600 keV, and (b) a garded, four-element, solar particle telescope for detecting protons from 0.5-100 MeV and alpha particles from 3.6-380 MeV, in 13 dfferentially-counted channels. Separation of protons from alpha particles is accomplished by an in-flight particle identifier. Two additional simplified telescopes are used to extend the angular coverage of the primary telescope, and a wide angle, high sensitivity scintillation detector is employed to monitor low intensity solar particle events. The design and operation of these instruments will be described.

An unusual gamma-ray burst event was observed on 5 March 1979 by nine different spacecraft. The position of the event has been accurately determined as r.a. = 5 h 25.95 min, dec = -66 deg 07.1 arcmin (epoch 1950.0), coincident with the location of the supernova remnant N49 in the Large Magellanic Cloud. The burst was of very high intensity, and if isotropic and located at the distance (approximately 55 kpc) of N49 had a peak luminosity of greater than 10 to the 44th erg/sec. Even more interesting is the obvious 8-s periodicity of the event, following the initial very intense outburst. The time history and power spectrum of this event as determined from Pioneer Venus Orbiter data is here reported.

This experiment is designed to provide continuous coverage of solar flare X-ray bursts and transient cosmic gamma-ray bursts. A proportional counter and a scintillation detector together cover the energy range from 5 to 228 keV with good sensitivity, large dynamic range, and high temporal resolution. This experiment provides data storage capability and good absolute timing so that in conjunction with similar experiments on other spacecraft, accurate source locations can be obtained for cosmic gamma-ray bursts.

The University of California at Berkeley (UCB)/Los Alamos Scientific Laboratory (LASL) gamma-ray burst experiment on board ISEE-3 is described and initial results of the experiment are presented. The instrument consists of an X-ray spectrometer originally designed to monitor solar emission continuously over the energy range 4.8-1264 keV with additional electronics to record data from the intense, short-lived gamma ray bursts at a very high rate and with good timing accuracy. Since the launch of ISEE-3 into a heliocentric orbit at the inner Lagrangian point in August, 1978, the UCB/LASL experiment has detected and recorded 12 gamma-ray bursts, including the unusually intense burst of November 4, 1978, which exhibited most of the classical gamma-ray burst characteristics, the intense burst of November 19, 1978, with an atypical time history and an unusually hard spectrum, and the extended burst of March 7, 1979. The spectral and temporal data demonstrate the diverse nature of the gamma-ray burst phenomenon, and are currently being combined with those of the Pioneer Venus Orbiter and Venera 11 and 12 to obtain accurate burst locations.

γ-ray burst spectra have often been fit by optically thin thermal bremsstrahlung. However, at the high temperatures implied by such fits (kT∼300 keV), the free–free cross-section is so much smaller than the Compton cross-section that Compton scattering might dominate the spectral formation processes. We have investigated the possibility that emission mechanisms based on Compton scattering can also fit the data. In particular, Monte Carlo calculations have been used to compare the γ-ray burst spectral data with black-body spectra which have undergone inverse comptonization by a much hotter, overlying plasma. The best-fit parameters suggest that the underlying black body is an X-ray source (kTBB∼2.4 keV) while the overlying, comptonizing plasma has kTe∼150 keV and a column density of 4 × 1024 electrons cm−2. Such a model would also naturally explain some of the unusual γ-ray burst spectra. In particular, a low energy (∼400 keV) cutoff in the spectrum of GB781119 can be explained as a Wien peak produced by a higher column density in the comptonizing region, and two-component spectra (for example, GB790329) can be explained by a lower column density.

There are currently two radiation mechanisms being considered for gamma‐ray bursts: thermal synchrotron and inverse comptonization. They are mutually exclusive because thermal synchrotron requires a magnetic field of ∼1012 Gauss, whereas inverse comptonization cannot produce a monotonic spectrum if the field is larger than 1011 and is too inefficient relative to thermal synchrotron unless the field is less than 109 Gauss. Neither mechanism can explain completely the observed characteristics of gamma‐ray bursts. However, we conclude that thermal synchrotron is more consistent with the observations if the sources are ∼40 kpc away, whereas inverse comptonization is more consistent if they are ∼300 pc away. Unfortunately, the source distance still is not known and, thus, the radiation mechanism still is uncertain.

The Los Alamos National Laboratory has provided energetic particle sensors for a variety of spacecraft at the geostationary orbit (36,000 km altitude). The sensor system called the Charged Particle Analyzer (CPA) consists of four separate subsystems. The LoE and HiE subsystems measure electrons in the energy ranges 30 to 300 keV and 200 to 2000 keV, respectively. The LoP and HiP subsystems measure ions in the ranges 100 to 600 keV and 0.40 to 150 MeV, respectively. A separate sensor system called the spectrometer for energetic electrons (SEE) measures very high-energy electrons (2 to 15 MeV) using advanced scintillator design. In this paper we describe the relationship of operational anomalies and spacecraft upsets to the directly measured energetic particle environments at 6.6 R/sub E/. We also compare and contrast the CPA and SEE instrument design characteristics with the next generation of Los Alamos instruments to be flown at geostationary altitudes.