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Gamma ray bursts display a wide range of temporal and spectral characteristics, but typically last several seconds and emit most of their energy in a low energy, gamma ray region. The burst sources appear to be isotropically distributed on the sky. Several lines of evidence suggest magnetic neutron stars as sources for bursts. A variety of energy sources and emission mechanisms are proposed.
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.
We show that gamma-ray bursts, observed by the Konus experiment on Venera 11 and Venera 12, from the three identified sources of short-term repeating bursts have a very low average V/V(max) of only 0.17 +/- 0.10 and are very important contributors to the low average V/V(max) of 0.43 +/- 0.03 found from the entire Konus burst sample. We further point out that the expected number of additional, as yet unidentified, repeating sources is quite sufficient to account for the low Konus average V/V(max), assuming that the average V/V(max) for bursts from all other sources is 0.5. Thus we suggest that the class of short-term repeating gamma-ray burst sources, as yet only partially identified, are a high-luminosity population that could be responsible for the values of average V/V(max) less than 0.5 also found by the BATSE, SMM, and other gamma-ray burst experiments. We note, however, that repeating bursts cannot as yet be identified solely by their spectral and temporal properties.
Several experiments are outlined which are designed for investigating cosmic gamma ray bursts; they form a research program that includes balloon experiments currently under development and satellite experiments proposed for future missions. The satellite proposals, in most cases, are either suggested for payloads that are presently defined as to experimental complement, or are for hypothetical spacecraft. Thus, these proposals at present serve to indicate, at least, the possible usefulness of various space missions for cosmic gamma-ray burst studies. Taken as a whole, this program of gamma ray burst experiments should improve on the present observations of the temporal variations, size spectrum, energy spectra, polarizations, directional locations and distribution of source directions, each by several orders of magnitude of improved resolution or of dynamic range.
The evidence for spectral features in gamma-ray bursts is summarized. As a guide for evaluating the evidence, the properties of gamma-ray detectors and the methods of analyzing gamma-ray spectra are reviewed. In the 1980's, observations indicated that absorption features below 100 kev were present in a large fraction of bright gamma-ray bursts. There were also reports of emission features around 400 kev. During the 1990's the situation has become much less clear. A small fraction of bursts observed with BATSE have statistically significant low-energy features, but the reality of the features is suspect because in several cases the data of the BATSE detectors appear to be inconsistent. Furthermore, most of the possible features appear in emission rather than the expected absorption. Analysis of data from other instruments has either not been finalized or has not detected lines.
The Burst and Transient Source Experiment (BATSE) on the Gamma Ray Observatory (GRO) is designed to study astronomical gamma ray sources and to provide better positional, spectral, and time resolution about these objects than has previously been possible from one experiment. The procedure to be used in the analysis of the gamma ray burst spatial distribution is presented. Data is input from BATSE via the Gamma Ray Burst Catalog (listing individual burst positions, flux values, and associated errors) and the Sky Sensitivity Map (which summarizes observational selection effects in table format). A FORTRAN program generates Monte Carlo burst catalogs, which are models to be compared to the actual distribution. The Monte Carlo models are then filtered through the Sky Sensitivity Map so that they suffer from the same selection effects as the actual catalog data. Additionally, each burst position is converted into a probability distribution to mimic BATSE positional sensitivity. The Burst Catalog, Monte Carlo burst catalog, and Sky Sensitivity Map are then passed onto an IDL program that compares the catalogs for statistical significance. The Sky Sensitivity Map is used to estimate how often each sky area is observed above the minimum flux level in question. Each burst found in this sky area is then weighted according to the frequency with which this sky area is observed. The catalogs are then compared via tests of homogeneity (based on their radial distributions) and isotropy (based upon their angular distributions). The results of the statistical comparisons along with graphs and charts of the summaries, are output from the IDL program for study.
Recent measurements and discoveries in gamma ray bursts and transients are reviewed including observations of the red shifted annihilation line in two kinds of slow transients (in 'classical' gamma ray bursts and in the unique 1979 March 5th event); of red shifted nuclear lines in a slow transient and in one gamma ray burst; and of the positions of precise source locations of gamma ray bursts and of the March 5th event, within the supernova remnant N49 in the Large Magellanic Cloud.
It has been suggested that the Burst and Transient Source Experiment (BATSE) gamma-ray burst catalog contains several groups of bursts clustered in space or in space and time, which provide evidence that a substantial fraction of the classical gamma-ray burst sources repeat. Because many of the bursts in these groups are weak, they are not directly detected by the Ulysses GRB experiment. We apply the network synthesis method to these events to test the repeating burst hypothesis. Although we find no evidence for repeating sources, the method must be applied under more general conditions before reaching any definite conclusions about the existence of classical gamma-ray burst repeating sources.
Moon Burst Energetics All-sky Monitor (MoonBEAM) is a CubeSat concept of deploying gamma-ray detectors in cislunar space to increase gamma-ray burst detections and improve localization precision with the timing triangulation technique. A gamma-ray instrument in cislunar orbit will have greatly reduced sky blockage compared to instruments in low Earth orbit. Working in conjunction with another instrument in low Earth orbit, MoonBEAM can also help constrain the arrival direction of the wavefront to an annulus on the sky by utilizing the light arrival times between the different orbits. This method has been demonstrated by the Interplanetary Gamma- Ray Burst Timing Network. However, delays in data downlink for instruments outside the Tracking and Data Relay Satellite network prevent rapid follow-up observations. We present here a gamma-ray CubeSat concept in Earth-Moon L3 halo orbit that is capable of faster response and provide a timing baseline for localization improvement. Such an instrument would aid in the gravitational wave follow-up observations in other wavelengths to identify the gamma-ray burst afterglow and kilonova emission. Reducing the region of interest makes identifying afterglows much faster, allowing for rapid on-source observations and monitoring of the rise and decay times. It will also prevent source confusion between two transients and enable robust association. A gamma-ray detection could also increase the confidence of a simultaneous but marginal gravitational wave signal, extending the detection horizon. MoonBEAM is a 12U CubeSat concept of deploying gamma-ray detectors in cislunar space to increase gamma-ray burst detections and improve localization precision with the timing triangulation technique. Such an instrument would probe the extreme processes in cosmic collision of compact objects and facilitate multi-messenger time-domain astronomy to explore the end of stellar life cycles and black hole formations.
It is shown here that any model for gamma-ray bursts in which the gamma-rays in the source are isotropic in a frame moving relativistically with respect to external observers has several advantageous traits: hard photons can escape more easily, the radiated energy per burst is reduced, and stellar back-heating is virtually eliminated. In addition, several other features of gamma-ray burst phenomenology must be reinterpreted: the burst source density, or alternatively, the recurrence time per source, is altered, and the physical interpretation of spectral features may also be substantially changed.
Spectral observations on nine of the eleven known cosmic gamma ray bursts detected from IMP-7 from late 1972 through 1973 are reported. All nine events were found to have average spectra ranging from 100 to 1100 keV. The hypothesis of a single greater than 100 keV average event spectrum for all gamma ray bursts (inferred from IMP-6 observations) was investigated. In addition to the mutual consistency of these 14 IMP events, all independently reported average event spectra fit the IMP-7 common spectrum. This observed lack of variability of the average event spectrum suggests that the production mechanism is tied to more rigidly fixed source parameters than most celestial or solar hard X-ray phenomena. Other characteristics of gamma ray bursts events, are discussed: a high-energy spectral tail with power-law index -2.5, a rising size spectrum from the earlier event rate, and a hint of clustering of event occurrence in time.
A search was conducted for short timescale (less than or approx. 10 s) faint X ray bursts in the complete 201,000,000 photon Einstein Observatory imaging proportional counter (IPC) data base. Some 73,000,000 events were extracted which were distributed among 11,230 observing intervals with an average observing time of approx. 1400 seconds. To exclude all sources, only those spatial pixels were searched with a count rate less than 6 x 10(exp -4) counts/sec sq arcmin. The mean count rate was much less. All files with less than 400 secs of observing time were excluded. The data was then binned into overlapping cells 4'.3 by 4'.3 by 10 secs in volume, and all the space-time cells were searched to locate cells with more than 5 counts in a 10 sec interval; i.e., a minimum flux enhancement of 50 over the mean rate for that point in space. Some 52 such events were found. An annulus was inspected around the event to see if it coincided with a general rise in the counter's event rate. Ten events did coincide with counter phenomena; these were associated with sunrise, sunset, or the approach of the South Atlantic Anomaly and were excluded. The remaining 42 events were then examined in an attempt to associate them with instrumental or environmental artifacts. The event occurrence times are uniformly distributed over the course of the mission and show no correlation with local solar time; their positions are randomly distributed in geographic, Galactic, and celestial coordinates. Their summed radial distribution is consistent with the point response function of the IPC for a soft source, indicating that the events are focused by the Observatory's mirror system. The integrated spectrum is, indeed, quite soft and is very similar to that of the cataclysmic variable star U Gem in outburst. Most of the events have risetimes of less than or approx. 1 sec and decay times ranging from 1 to 20 sec. Their positions are not associated with any class of catalogued objects. Several possible origins are discussed for the events; if they are associated with gamma ray bursters, they provide interesting constraints on the space density and repetition rate of the underlying source population.
The current status of energy sources in models of gamma-ray bursts is examined. Special emphasis is placed on the thermonuclear flash model which has been the most developed model to date. Although there is no generally accepted model, if the site for the gamma-ray burst is on a strongly magnetized neutron star, the thermonuclear model can qualitatively explain the energetics of some, but probably not all burst events. The critical issues that may differentiate between the possible sources of energy for gamma-ray bursts are listed and briefly discussed.
We present the results from the first long-term search for nonburst gamma-ray emission from the positions of 70 intense, well-localized bursts. Using the BATSE occultation technique, designed for monitoring of discrete sources, these burst positions were measured in the energy range of approximately 15 keV to 1.8 MeV over a 112 day interval during 1991. None of these 70 locations exhibited detectable emission at or above the level of approximately 5 x 10(exp -9) ergs cm(exp -2) s(exp -1) during the 112 day interval. This level is approximately 1000 times less than the typical intensity of the burst associated with the given location. In addition, 35 intense gamma-ray bursts detected by BATSE were examined in a five day interval centered on the time of detection. We find no compelling evidence that these bursts emit preburst emission or display prompt postburst emission at a level of approximately 5 x 10(exp -9) ergs cm(exp 2) s(exp -1) on timescales of approximately 1 hr or longer. The lack of detectable long-term emission or preburst and postburst emission from the positions of gamma-ray bursts has important consequences for a variety of burst production models.
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We report evidence from the Third BATSE Gamma-Ray Burst Catalog that long (T(sub 90) greater than 10 s) and short (T(sub 90) less than 10 s) gamma-ray bursts (GRBS) represent distinct source populations. Their spatial distributions are significantly different, with long bursts having = 0.282+/- 0.014 but short bursts having = 0.385 +/- 0.019, differing by 0.103 +/- 0.024, significant at the 4.3 or level. This implies different spatial origin and physical processes for long and short bursts. Long bursts may be explained by accretion-induced collapse. Short bursts require another mechanism, for which we suggest neutron star collisions. These are capable of producing neutrino bursts as short as a few milliseconds, consistent with the shortest observed timescales in GRBS. We briefly investigate the parameters of clusters in which neutron star collisions may occur, and discuss the nuclear evolution of expelled and accelerated matter.
Gamma-ray bursts are among the most fascinating occurrences in the cosmos. They are thought to be the birth cries of black holes throughout the universe. The NASA swift mission is an innovative new multiwavelength observatory designed to determine the origin of bursts and use them to probe the early Universe. Swift is now in orbit since November 20, 2004 and all hardware is performing well. A new-technology wide-field gamma-ray camera is detecting a hundred bursts per year. sensitive narrow-field X-ray and uv/optical telescopes, built in collaboration with UK and Italian partners, are pointed at the burst location in 50-100 sec by an autonomously controlled "swift" spacecraft. For each burst, arcsec positions are determined and optical/UV/X-ray/gamma-ray spectrophotometry performed. Information is also rapidly sent to the ground to a team of more than 50 observers at telescopes around the world. The first year of findings from the mission will be presented. There has been a break-through in the longstanding mystery of short GRBs; they appear to be caused by merging neutron stars. High redshift bursts have been detected leading to a better understanding of star formation rates and distant galaxy environments. GRBs have been found with giant X-ray flares occurring in their afterglow.
Gamma-ray bursts are among the most fascinating occurrences in the cosmos. They are thought to be the birth cries of black holes throughout the universe. The NASA Swift mission is an innovative new multiwavelength observatory designed to determine the origin of bursts and use them to probe the early Universe. Swift is now in orbit after a beautiful launch on November 20, 2004. A new-technology wide-field gamma-ray camera detects more than a hundred bursts per year. Sensitive narrow-field X-ray and UV/optical telescopes, built in collaboration with UK and Italian partners, are pointed at the burst location in 20 to 70 sec by an autonomously controlled "swift" spacecraft. For each burst, arcsec positions are determined and optical/UV/X-ray/gamma-ray spectrophotometry performed. Information is also rapidly sent to the ground to a team of more than 50 observers at telescopes around the world. The first year of findings from the mission will be presented. The long-standing mystery of short GRBs has been solved, and the answer is the most interesting possible scenario. High redshift bursts have been detected leading to a better understanding of star formation rates and distant galaxy environments. GRBs have been found with giant X-ray flares occurring in their afterglow. These, and other topics, will be discussed.