Observations of cosmic gamma-ray bursts

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.Work performed under the auspices of the United States Energy Research and Development Administration.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

The microsecond variability of cosmic gamma-ray bursts

The possibility of microsecond flares from cosmic gamma-ray sources is considered taking into account theoretical models. Several observations effects resulted from these flares are also presented.     

The statistical properties of cosmic gamma-ray bursts

Recent localizations of cosmic gamma-ray bursts (GBRs) are studied taking into account the estimations of their intensities. The increase of the avergage intensity of GRBs along galactic longitudesl=220–260° andl=40–80° is found. It may be associated with the Orion spiral arm and the central region of the Galaxy. Corresponding enhancement of the number of intense GRBs for directions along the Orion arm is also found. Astrophysical conclusions from this possible galaxy-scale distribution of GRBs are discussed.     

Ulysses observations of cosmic gamma-ray bursts

We review the current status of the Ulysses mission and summarize the results to date of the GRB experiment. This instrument detects bursts at the rate of about one every 3.5 days, and the localization data are being disseminated rapidly via the BACO-DINE and NMSU networks. The mission should operate through 2001, and future missions to Mars starting in 1996 will complete the 3rd Interplanetary Network.     

Annihilation radiation in cosmic gamma-ray bursts

The emission features observed in the energy spectra of cosmic gamma-ray bursts imply the existence of two radiation components of comparable intensity. The softer component is similar to the continua of featureless bursts. The fast decrease in the intensity of this radiation with increasing photon energy is apparently due to the neutron star's magnetosphere being opaque to hard photons because of the formation of electron-positron pairs in single- (,B) and two-photon (,), processes. The hard component originates from the annihilation of electron-positron pairs, its spectrum representing a broad line with an extended power-law wing. Such a shape of the spectrum is apparently due to either thermal broadening in a source with a spatially inhomogeneous and rapidly time-varying plasma temperature, or nonthermal energy distribution of particles in their motion along the magnetic field lines. It is assumed that the sources of these components are spatially separated, the annihilation radiation escaping from the polar regions of a strongly magnetized neutron star in a collimated beam without appreciable attenuation.     

On the nature of cosmic gamma-ray bursts

Photoelectric and spectrographic observations of the Be star Eri made during five nights indicate that the luminosity variations have a different periodicity from that of radial velocity variations. This result seems to show that, at least during our observing time, the observed variability has not to be ascribed to a pulsation of the star.     

On the origin of gamma-ray bursts

A model for gamma-ray bursters which naturally explains the main observational features of GRB is proposed. The GRB turned out to be linked by evolution with a well-known type of astrophysical object.     

Gamma-ray bursts, ultra-high-energy cosmic rays, and cosmic gamma-ray background

We argue that gamma-ray bursts (GRBs) may be the origin of the cosmic gamma-ray background radiation observed in the GeV range. It has theoretically been discussed that protons may carry a much larger amount of energy than electrons in GRBs, and this large energy can be radiated in the TeV range by synchrotron radiation of ultra-high-energy protons ( 10²⁰ eV). The possible detection of GRBs above 10 TeV suggested by the Tibet and HEGRA groups also supports this idea. If this is the case, most of TeV gamma-rays from GRBs are absorbed in intergalactic fields and eventually form GeV gamma-ray background, whose flux is in good agreement with the recent observation.     

United classification of cosmic gamma-ray bursts and their counterparts

This paper establishes united classification of gamma-ray bursts and their counterparts on the basis of measured characteristics: photon energy E and emission duration T. We find that the interrelation between these characteristics is such that as the energy increases, the duration decreases (and vice versa). The given interrelation reflects the nature of the phenomenon and forms the E–T diagram, which represents a natural classification of all observed events in the energy range from about 10⁹ to 10⁻⁶ eV and in the corresponding interval of durations from about 10⁻² up to 10⁸ s. The proposed classification results from our findings, which are principal for the theory and practical study of the phenomenon.     

Analysis of narrow spikes in two cosmic gamma-ray bursts

An analysis has been made of the narrow intensity spikes occurring in two cosmic gammaray bursts which were observed with a fast time resolution germanium spectrometer on board the low altitude polar orbiting satellite 1972-076B (Imhofet al., 1974a, 1975; Nakanoet al., 1974a). The bursts on 18 December, 1972 and 21 July, 1973, were also recorded on the Vela satellite system. The durations of three spikes were observed to be 0.06 s with the limits being 0.03 to 0.10 s. Four other narrow spikes were measured to have time width limits between 0.001 and 0.10 s. An additional eight spikes had widths less than 0.9 s. During the spikes, the gamma-ray intensities increased by factors of 2 to 10, with a median value of 3. These and other characteristics of the fast time structure are presented.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Catalog of cosmic gamma-ray bursts from the KONUS experiment data

Data are presented on the temporal structure, fluxes, energy spectra and coordinates of the sources of gamma-ray bursts detected in the KONUS experiment on Venera 11 and Venera 12 space probes in the period September 1978 to May 1979. The statistical distributions of gamma bursts in duration, intensity, and peak power, as well as the distribution of the burst sources over the celestial sphere presented are based on the updated KONUS information obtained until February 1980.     

Oscillation structure of gamma-ray bursts and their possible origin

It is hypothesized that thermonuclear burning of the matter from the envelope of a massive compact star accreting onto a hot neutron star produced by spherically symmetric collapse of a stellar iron core can proceed in oscillation mode (much as is the case during thermal explosions of carbon-oxygen cores in lower mass stars). Local density oscillations near the neutron-star surface can generate shock waves; in these shocks, the electron-positron plasma is stratified from the remaining matter, and shells of an expanding relativistic fireball with an oscillation time scale in cosmological gamma-ray bursts (GRBs) of ～10^?2 s are formed. It is pointed out that the GRB progenitors can be nonrotating massive Wolf-Rayet (WR) stars whose collapse, according to observational data, can proceed without any substantial envelope ejection.     

Selection effects on the size and frequency distribution of cosmic gamma-ray bursts

We investigate selection effects on the size and frequency relation (logN-logS curve) of cosmic gamma-ray bursts. After analyzing the published data, we find an indication that an effect is caused by the different time profiles of the bursts. The effect is important for small bursts and causes significant changes in the logN-logS curve. in order to avoid this selection effect, we mention that it is essential to use the logN-logP relation of the logN-logS relation, as already suggested by other authors. Here,P is the peak flux of the burst, which is free from bias due to the difference in time profiles. After an analysis of the published data, we find a distribution nearP ^–3/2 in the range above a peak flux of 40 counts/0.25 s.We also show that the relation between the logN-logP curve and the observed celestial distribution for the bursts can easily be explained by our general arguments on a disk-like spatial distribution of burst sources in the Galaxy.     

Search for ultrahigh-energy cosmic gamma-ray bursts on the Baksan underground scintillation telescope

Based on data from the Baksan underground scintillation telescope (BUST) for the period 2001–2004, we searched for cosmic gamma-ray bursts (GRBs) at primary photon energies of 0.5 TeV or higher. We obtained constraints on the rate of bursts with durations of 1–10 s for fluences within the range 4.6 × 10⁻³-1.8 × 10⁻² erg cm⁻² in the declination band 30° ≤ δ ≤ 80°. We searched for ultrahigh-energy gamma rays from GRBs detected on spacecraft during and within ±2 h of the burst. No statistically significant excesses above the background of random coincidences were found. The derived constraints on the ultrahigh-energy gamma-ray fluence during GRBs lie within the range 4.6 × 10⁻³-3.7 × 10⁻² erg cm⁻².     

Superlong gamma-ray bursts

We searched for anomalously long gamma-ray bursts (GRBs) in the archival records of the Burst and Transient Sources Experiment (BATSE). Ten obvious superlong (>500 s) GRBs with almost continuous emission episodes were found. Nine of these events were known from the BATSE catalog, but five had no duration estimates; we found one burst for the first time. We also detected events with emission episodes separated by a long period of quiescence (up to ～1000 s) with a total duration of 1000–2000 s. In the latter case, we cannot reach an unequivocal conclusion about a common origin of the episodes due to the BATSE poor angular resolution. However, for most of these pairs, the probability of independent GRBs coinciding is much lower than unity, and the probability that all of these are coincidences is ～10^?8. All of the events have a hardness ratio (the ratio of the count rates in different energy channels) typical of GRBs, and their unique duration is unlikely to be related to their high redshifts. Superlong bursts do not differ in their properties from typical long (>2 s) GRBs. We estimated the fraction of superlong GRBs (>500 s) among the long (>2 s)GRBs in the BATSE sample with fluxes up to 0.1 ph cm^?2 s^?1 to be between 0.3 and 0.5%, which is higher than the estimate based on the BATSE catalog.     

Periodicities in gamma-ray bursts

One of the most recent discoveries on gamma-ray bursts was the fact that one, or possibly two, of them had a pulsed emission with a periodicity of 8 and 4 s. The intensity of successive peaks decreased exponentially with time. We can speculate that the energy range and/or the lack of sensitivity are probably the reasons why periodicities were not discoverded in some other bursts. In fact, many time-histories show a double or multiple peak structure. If they were periodic, the distance between successive peaks would be equal to the period. Based on this extremely simplified assumption, a search on all available gamma-ray burst time-histories shows that most the periodicities above 1 s should be contained within an interval between 1 and 9 s, with a maximum at approximately 3 s.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November, 1979.     

The neutron-star oscillations model for gamma-ray bursts

Exact analytic expressions for the vacuum electromagnetic fields produced by an oscillating magnetized sphere are obtained. The solutions are analysed for various modes of pulsation and for low-order multipole magnetic moments. Within the context of neutron star oscillations, the possibility of gamma-ray generation is discussed. It is shown that the radial pulsations provide an efficient mechanism for generation of gamma-radiation and electron-positron pairs in some regions around the neutron star. For this, the non-vanishing quadrupole magnetic moment oblique to the dipole moment is required. The model for gamma-ray bursts that we propose is briefly considered.     

Cosmological models of gamma-ray bursts

We review models of cosmological gamma-ray bursts (GRBs). The statistical and -ray transparency issues are summarized. Neutron-star and black-hole merger scenarios are described and estimates of merger rates are summarized. We review the simple fireball models for GRBs and the recent work on non-simple fireballs. Alternative cosmological models, including models where GRBs are analogs of active galactic nuclei and where they are produced by high-field, short period pulsars, are also mentioned. The value of neutrino astronomy to solve the GRB puzzle is briefly reviewed.