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The research program in gamma-ray burst astronomy at the NASA/Marshall Space Flight Center is described. Large-area scintillation detector arrays have been flown on high-altitude balloons, and an array is being developed for the Gamma-Ray Observatory. The design of these detectors is described along with results obtained from previous balloon flights.
The Pioneer Venus Orbiter gamma burst detector is an astrophysics experiment for monitoring cosmic gamma-ray bursts. It is included in this planetary mission to provide a long baseline for accurately locating the sources of these bursts in order to identify them with specific astronomical objects. Responses to 14 gamma-ray burst events were examined; these events were verified from data acquired by other systems. Preliminary locations are proposed for three events, based on data from the Pioneer Venus Orbiter, ISEE C, and Vela spacecraft. These locations will be improved, and additional locations will be determined by including in the analyses data from Helios B and the Russian Venera 11, Venera 12, and Prognoz 7 spacecraft.
A cosmic gamma-ray burst is described which was observed from a balloon with a double-scatter gamma-ray telescope. The observed burst had a rise time of about 0.15 sec, a duration of 0.11 sec, and variations on a time scale of 5 msec. The variations are shown to imply a maximum source dimension of 1500 km. The integral energy distribution of the burst is determined along with the total energy above 0.5 MeV. The direction of the source is found, with 90% confidence, to be limited to a circle with a radius of 25 deg centered at +39 deg galactic latitude and 40 deg galactic longitude. Two additional candidate bursts observed with the same telescope are briefly discussed.
The most important observational characteristics of gamma-ray bursts are reviewed, with emphasis on X-ray and gamma-ray data. The observations are used to derive some basic properties of the sources. The sources are found to be isotropically distributed; the burster population is limited in space, and the edge of the distribution is visible.
The variability in multi-pulse gamma-ray bursts (GRBs) may help to reveal the mechanism of underlying processes from the central engine. To investigate whether the self-organized criticality (SOC) phenomena exist in the prompt phase of GRBs, we statistically study the properties of GRBs with more than 3 pulses in each burst by fitting the distributions of several observed physical variables with a Markov Chain Monte Carlo approach, including the isotropic energy E iso , the duration time T, and the peak count rate P of each pulse. Our sample consists of 454 pulses in 93 GRBs observed by the CGRO/BATSE satellite. The best-fitting values and uncertainties for these power-law indices of the differential frequency distributions are: α$^{d}_{E}$ = 1.54±0.09, α$^{d}_{T}$ = 1.82±$^{+0.14}_{0.15}$ and α$^{d}_{P}$ = 2.09$^{+0.18}_{–0.19}$, while the power-law indices in the cumulative frequency distributions are: α$^{c}_{E}$ = 1.44$^{+0.08}_{–0.10}$, α$^{c}_{T}$ = 1.75$^{+0.141}_{–0.13}$ and α$^{c}_{P}$ = 1.99$^{+0.16}_{–0.19}$. We find that these distributions are roughly consistent with the physical framework of a Fractal-Diffusive, Self-Organized Criticality (FD-SOC) system with the spatial dimension S = 3 and the classical diffusion β=1. The results of this study support that the jet responsible for the GRBs should be magnetically dominated and magnetic instabilities (e.g., kink model, or tearing-model instability) lead the GRB emission region into the SOC state.
The 'event' that triggers a gamma-ray burst cannot last for more than a few seconds. This is, however, long compared with the dynamical timescale of a compact stellar-mass object (approximately 10 (exp-3) s). Energy is assumed to be released as an outflow with high mean Lorentz factor Gamma. But a compact stellar-mass collapse or merger is, realistically, likely to generate a mass (or energy) flux that is unsteady on some timescales in the range 10(exp -3) - 10 s. If Gamma fluctuates by a factor of approximately 2 around its mean value, relative motions within the outflowing material will themselves (in the comoving frame) be relativistic, and can give rise to internal shocks. For Gamma approximately 10(exp 2), the resultant dissipation occurs outside the 'photosphere' and can convert a substantial fraction of the overall outflow energy into nonthermal radiation. This suggests a mechanism for cosmological bursts that demands less extreme assumptions (in respect of Gamma-values, freedom from baryonic contamination, etc.) than earlier proposals.
The celestial source location of the November 19, 1978, intense gamma ray burst has been determined from data obtained with the interplanetary gamma-ray sensor network by means of long-baseline wave front timing instruments. Each of the instruments was designed for studying events with observable spectra of approximately greater than 100 keV, and each provides accurate event profile timing in the several millisecond range. The data analysis includes the following: the triangulated region is centered at (gamma, delta) 1950 = (1h16m32s, -28 deg 53 arcmin), at -84 deg galactic latitude, where the star density is very low and the obscuration negligible. The gamma-ray burst source region, consistent with that of a highly polarized radio source described by Hjellming and Ewald (1981), may assist in the source modeling and may facilitate the understanding of the source process. A marginally identifiable X-ray source was also found by an Einstein Observatory investigation. It is concluded that the burst contains redshifted positron annihilation and nuclear first-excited iron lines, which is consistent with a neutron star origin.
Calculations are made of the approximate characteristics of the reprocessed optical radiation resulting from the absorption of a gamma-ray burst by a nearby star. The overall reprocessing time scale, including contributions from gamma-ray transfer and diffusion of the optical radiation, is estimated. It is noted that diffusive cooling occurs by either a 'self-similar' or a 'transparency' wave of postabsorption stellar surface temperatures greater or less than 10,000 K, respectively. Depending on the combination of stellar and gamma-ray burst properties, the reprocessing time scale can vary from the duration of the gamma-ray burst (approximately 1 s) to the maximum calculated cooling time (approximately 1000 s). Calculations for close binary systems are made of the number of optical photons per square centimeter expected at earth as a function of the observed gamma-ray burst fluences and likely distances. It is predicted that, if all gamma-ray bursters are in close binaries, the number of detections per year by an all sky monitor of 200 photons per sq cm sensitivity will be about 100.
Gamma-ray bursts display a wide range of temporal and spectral characteristics, but typically last several seconds and emit most of their energy in the 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 were proposed.
Twenty-one gamma-ray bursts and 68 solar flares in the hard X-ray range were detected on Venera-11 and Venera-12 space probes during the initial 50-day observation period. Major characteristics of the equipment used and preliminary data on the temporal structure and energy spectra of the gamma-ray bursts are considered. The pattern of gamma-ray burst frequency distribution vs. intensity, N(S), is established.
Although the association of gamma-ray bursts (GRBs) with massive stellar death is on firm footing, the nature of the progenitor system and the key ingredients required for a massive star to produce a GRB remain open questions. Here, we investigate the evolution of a 15–25M ⊙ massive star with a 10–15M ⊙ black hole (BH) using Modules for Experiments in Stellar Astrophysics. We quantify companion-influenced angular-momentum evolution over a stellar lifetime for orbital periods where tides are significant, varying stellar and BH masses, initial stellar spin, and accretion and dynamo prescriptions while tracking mass loss and angular momentum. The final spin is set by tidal torques versus stellar winds. For binaries that initially avoid Roche-lobe overflow, tides can spin up the star, but a late-stage expansion can drive tidal stripping; associated mass and angular-momentum loss can suppress spin-up. We find that massive-star BH binaries at comparable mass ratios may be potential GRB progenitors for short orbital periods (∼20–5 × 10 2 days) and long orbital periods (∼2 × 10 3 –4 × 10 3 days), where our suite of lifetime simulations reveals a favored parameter space with negligible mass loss and enough spin angular momentum to power a GRB jet. For initially nonrotating stars, this provides a lower limit on the final spin above a threshold estimate consistent with forming a postcollapse BH mass of 5–10M ⊙ with a spin parameter ≥ 0.5. For initially rapidly rotating stars, tidal interactions may sustain high spin when mass loss is negligible because the binary is not tidally synchronized.
A cosmic gamma ray burst at about 1058:21 UT on April 27, 1972, was detected with an omnidirectional scintillation counter on Apollo 16. The event whose energy was approximately 0.0002 erg/sq cm, was pulse height analyzed in 512 channels over the 0.067 less than or equal to E less than or equal to 5 MeV range, and shows a complex, multipeak structure on a 300 msec time scale. The burst was also measured by a collimated X-ray detector on Apollo 16, giving spectral data in the 2 to 8 keV range which, together with a simultaneous observation by Vela 6A, allows a directional determination.
A systematic classification of gamma-ray bursts is considered on the basis of temporal and spectral burst features. Three categories of bursts are proposed, and the category of classical (or normal) bursts is subdivided into three classes: very brief, doublet/quasi-periodic, and long irregular. Apparent correlations are discussed between emission lines and the doublet/quasi-periodic class and between absorption lines and the long irregular class.
The intrinsic luminosity, erg/s, in a gamma ray burst is calculated using two assumptions: first, the power is independent of the burst duration, and, second, the spatial distribution of the bursters is similar to the spatial distribution of the pulsars. It is found that the experimentally determined distribution of burst durations is significantly biased by the observational selection effects. It is shown that a typical luminosity of 5 x 10 to the 35th erg/s can adequately reproduce the burst (log N, log S) curve, an isotropic latitude distribution, and the experimentally determined distribution of durations.
The emission mechanisms of cosmic gamma-ray bursts are reviewed. In particular, the thermal synchrotron model is discussed as the most viable mechanism for the majority of the continuum emission. Within this framework various information about the source region can be extracted. The picture that emerges is that of a hot (kT = .2 - 1.0 sq mc), thin sheet of dense pair-dominated plasma emitting via cyclo-synchrotron radiation in a strong magnetic field (B approximately one-hundred billion to one trillion gauss). Speculations on the origin and structure of this sheet are attempted. The problem of high-energy photons above pair production threshold escaping from the source is also considered.
The pair annihilation radiation in gamma-ray bursts is seen as broad lines with extended hard wings. This radiation is suggested to escape in a collimated beam from magnetic polar regions of neutron stars.
A phenomenology for gamma ray burst time histories is outlined. Order of their generally chaotic appearance is attempted, based on the speculation that any one burst event can be represented above 150 keV as a superposition of similarly shaped increases of varying intensity. The increases can generally overlap, however, confusing the picture, but a given event must at least exhibit its own limiting characteristic rise and decay times if the measurements are made with instruments having adequate temporal resolution. Most catalogued observations may be of doubtful or marginal utility to test this hypothesis, but some time histories from Helios-2, Pioneer Venus Orbiter and other instruments having one-to several-millisecond capabilities appear to provide consistency. Also, recent studies of temporally resolved Solar Maximum Mission burst energy spectra are entirely compatible with this picture. The phenomenology suggested here, if correct, may assist as an analytic tool for modelling of burst processes and possibly in the definition of burst source populations.
The emission mechanisms of cosmic gamma-ray bursts are reviewed. In particular, the thermal synchrotron model is discussed as the most viable mechanism for the majority of the continuum emission. Within this framework various information about the source region can be extracted. The picture that emerges is that of a hot (kT = .2 - 1.0 sq mc), thin sheet of dense pair-dominated plasma emitting via cyclo-synchrotron radiation in a strong magnetic field (B approximately one-hundred billion to one trillion gauss). Speculations on the origin and structure of this sheet are attempted. The problem of high-energy photons above pair production threshold escaping from the source is also considered.