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At least 1,279 records · Page 71
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NASA NTRS1991

An observation of the Galactic center region with the HEXAGONE high resolution gamma-ray spectrometer

The Galactic center region was observed for 6 hours on May 22, 1989 from a high altitude balloon with the HEXAGONE high resolution gamma-ray spectrometer. The instrument had a 285 sq/cm array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18 deg FWHM field of view. The 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of Al-26 were observed to have fluxes of 8.9 x 10 exp -4 and 1.9 x 10 exp -4 ph/cm-s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of -2.6. In the 120-250 keV band the spectrum contains a broad linelike feature. This is interpreted as the result of Compton backscattering of about 511 keV photons from a compact source of electron-positron annihilation radiation.

Matteson, J.↗
NASA NTRS1991

The theory and diagnostic promise of supernova gamma-rays

The diagnostic power of gamma-ray and X-ray observations of supernovae of all types is reviewed. The basic features of supernova gamma-ray emission and the beginnings of a program to characterize the entire family of supernovae in gamma light are outlined.

Burrows, Adam↗
NASA NTRS1992

Self-absorption theory applied to rocket measurements of the nitric oxide (1, 0) gamma band in the daytime thermosphere

Observations of the UV fluorescent emissions of the NO (1, 0) and (0, 1) gamma bands in the lower-thermospheric dayglow, made with a sounding rocket launched on March 7, 1989 from Poker Flat, Alaska, were analyzed. The resonant (1, 0) gamma band was found to be attenuated below an altitude of about 120 km. A self-absorption model based on Holstein transmission functions was developed for the resonant (1, 0) gamma band under varying conditions of slant column density and temperature and was applied for the conditions of the rocket flight. The results of the model agreed with the measured attenuation of the band, indicating the necessity of including self-absorption theory in the analysis of satellite and rocket limb data of NO.

Eparvier, F. G.↗
NASA NTRS1987

Solar flare nuclear gamma rays and interplanetary proton events

We compared flare gamma ray line (GRL) events and solar energetic proton (SEP) events for the period from Feb. 1980 - Jan. 1985 and substantiated earlier results showing a lack of correlation between gamma-ray-producing ions and interplanetary protons. This poor correlation results primarily from several large SEP events that originated in flares without detectable gamma ray emission. The converse case of GRL events unassociated with SEP events is rare. We present evidence which suggests that the ratio of trapped to escaping protons in GRL/SEP flares depends on the spatial scale size of the flare. We affirm the result of Bai and Dennis (1985) that GRL flares are generally accompanied (75 percent) by metric Type 2 bursts.

Cliver, E. W.↗
NASA NTRS1992

Study of SMM flares in gamma-rays and neutrons

This report summarizes the results of the research supported by NASA grant NAGW-2755 and lists the papers and publications produced through the grant. The objective of the work was to study solar flares that produced observable signals from high-energy (greater than 10 MeV) gamma-rays and neutrons in the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS). In 3 of 4 flares that had been studied previously, most of the neutrons and neutral pions appear to have been produced after the 'main' impulsive phase as determined from hard x-rays and gamma-rays. We, therefore, proposed to analyze the timing of the high-energy radiation, and its implications for the acceleration, trapping, and transport of flare particles. It was equally important to characterize the spectral shapes of the interacting energetic electrons and protons - another key factor in constraining possible particle acceleration mechanisms. In section 2.0, we discuss the goals of the research. In section 3.0, we summarize the results of the research. In section 4.0, we list the papers and publications produced under the grant. Preprints or reprints of the publications are attached as appendices.

Dunphy, Philip P.↗
NASA NTRS1992

GAMCIT: A gamma ray burst detector

The origin of celestial gamma ray bursts remains one of the great mysteries of modern astrophysics. The GAMCIT Get-Away-Special payload is designed to provide new and unique data in the search for the sources of gamma ray bursts. GAMCIT consists of three gamma ray detectors, an optical CCD camera, and an intelligent electronics system. This paper describes the major components of the system, including the electronics and structural designs.

Surka, Derek M.↗
NASA NTRS1993

The FNAS Gamma Ray Observatory

Highly stressed and nonpotential magnetic fields in solar active region are believed to be the site and source of M or X class flares which release as much as 10(exp 32) ergs in a periods of 10 to 1000 s with intense emission detected over most of the electromagnetic spectrum. Such M or X class flares are the most likely gamma-ray emission candidates since sheared magnetic field is the means of energy storage for flare events. We have studied the correlation between 'nonpotentiality' parameters such as 'shear angle' and maximum magnetic field strength with the occurrences gamma-ray events. Also the angle of deviation of the magnetic field from the line of sight was studied since the bremsstrahlung gamma-ray beam produced by relativistic ions and electrons is highly directional in the forward direction and required non-vertical field-lines for the detection of the disk-center events.

Emslie, A. Gordon↗
NASA NTRS1993

Gamma Ray/neutron Spectrometers for Planetary Elemental Mapping

Los Alamos has designed gamma ray and neutron spectrometers for Lunar Scout, two robotic missions to map the Moon from 100 km polar orbits. Knowledge of the elemental composition is desirable in identifying resources and for geochemical studies and can be obtained using gamma ray and neutron spectrometers. Measurements with gamma ray and neutron spectrometers complement each other in determining elemental abundances in a planet's surface. Various aspects of the instruments are discussed.

Reedy, R. C.↗
NASA NTRS1992

Tidal heating and mass loss in neutron star binaries - Implications for gamma-ray burst models

A neutron star in a close binary orbit around another neutron star (or stellar-mass black hole) spirals inward owing to gravitational radiation. We discuss the effects of tidal dissipation during this process. Tidal energy dissipated in the neutron star's core escapes mainly as neutrinos, but heating of the crust, and outward diffusion of photons, blows off the outer layers of the star. This photon-driven mass loss precedes the final coalescence. The presence of this eject material impedes the escape of gamma-rays created via neutrino interactions. If an e(+) - e(-) fireball, created in the late stages of coalescence, were loaded with (or surrounded by) material with the mean column density of the ejecta, it could not be an efficient source of gamma-rays. Models for cosmologically distant gamma-rays burst that involve neutron stars must therefore be anisotropic, so that the fireball expands preferentially in directions where the column density of previously blown-off material is far below the spherically averaged value which we have calculated. Some possible 'scenarios' along these lines are briefly discussed.

Meszaros, P.↗
NASA NTRS1992

Are repeaters responsible for gamma-ray burst average V/V(max) less than 0.5?

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.

Lingenfelter, R. E.↗
NASA NTRS1992

Radiation transfer in gamma-ray bursts

We have calculated gamma-ray radiative transport in regions of high-energy density, such as gamma-ray burst source regions, using a discrete ordinate, discrete energy group method. The calculations include two-photon pair production and annihilation, as well as three-photon annihilation. The radiation fields itself acts as an absorbing medium, and the optical depth depends on its intensity, so the problem is intrinsically nonlinear. Spherical divergence produces effective collimation of the flux. At high optical depth the high energy (E is greater than 1 MeV) portion of the emergent spectrum assumes a nearly universal form. An approximate limit is derived for the high-energy flux from a gamma-ray burst source region of given size, and the implications of this limit for the distance to the 1979 March 5 event are briefly discussed. We discuss more generally the problem of very luminous bursts, and implications of Galactic halo distances for flare models.

Carrigan, B. J.↗
NASA NTRS1992

Upper limit on the steady emission of the 2.223 MeV neutron capture gamma-ray line from the sun

A search for steady emission of the 2.223 MeV gamma-ray line arising from the direct capture reaction 1H(n, gamma)2H is presented on the basis of an analysis of SMM data. The upper limits for continuous 2.223 MeV gamma-ray line emission from the active and inactive sun, as established in the present work, are about two orders of magnitude less than previously published results. These findings for inactive periods also suggest that small flares (or a continuous acceleration mechanism) are unlikely to be the source of the heating of the quiet solar corona. It is concluded that the power in accelerated nuclei during inactive periods falls short of the coronal heating requirement by about four orders of magnitude unless there is a large and as yet unobserved population of nuclei at energies below 1 MeV. The energy release in other forms associated with the acceleration process falls short of the heating requirement by about two orders of magnitude if this energy release is in the same proportion to the power in accelerated nuclei as is estimated from observed flares.

Harris, M. J.↗
NASA NTRS1992

Gamma-ray line afterglow from burst environments

We consider photoerosion and direct pair production in a medium surrounding a gamma-ray burst. The resulting secondary gamma-rays may provide diagnostic tools of these environments and, in turn, of the nature of the bursters themselves. In some instances short-lived nuclides are formed; the beta-delayed gamma-rays produced from their decays provide the signatures of the photoerosion. In addition, annihilation radiation produced from positrons resulting from direct pair production is related to the plasma conditions in the medium. We investigate the plausibility of detecting the various radiations. Under extremely favorable conditions, the photoerosion afterglow might be detectable with the present generation of detectors. However, the positron annihilation line should be detectable under a fairly wide range in the conditions of the medium.

Fencl, H. S.↗
NASA NTRS1992

Gamma-ray bursts from colliding neutron stars

Three distance scales to gamma-ray bursters are discussed: about 300 pc, about 2 - 50 kpc, and about 1 Gpc, corresponding to the Galactic disk, Galactic halo, and extragalactic origin. No compelling evidence is found in favor of any of them. The BATSE experiment on GRO should determine the distance scale by determining the angular distribution of very weak bursts. The rate of collisions between the neutron stars is about 0.0001/yr in our Galaxy, and about 10 exp 5/yr within the Hubble distance. The collisions are the final phases of binary orbit decay driven by gravitational radiation, and may produce gamma-ray bursts detectable at extra-galactic distances. If strange stars exist then their collisions must release about 10 exp 49 erg in gamma-rays over about 10 seconds. Such events should be detectable out to about 1 Gpc with the current instruments.

Paczynski, Bohdan↗
NASA NTRS1992

Bulk relativistic motion in gamma-ray bursts

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.

Krolik, Julian H.↗
NASA NTRS1992

The Ptolemaic gamma-ray burst universe

The BATSE experiment on GRO has demonstrated the isotropic arrival directions and flat log N vs log S of cosmic gamma-ray bursts. These data are best explained if the burst sources are distributed throughout an extended spherical galactic halo, as previously suggested by Jennings. The halo's radius is at least 40 kpc, and probably is more than 100 kpc. I consider possible origins of this halo, including primordial formation and neutron stars recoiling from their birthplaces in the galactic disk. A simple geometrical model leads to a predicted relation between the dipole and quadrupole anisotropy. I suggest that neutron stars born with low recoil become millisecond pulsars, while those born with high recoil become the sources of gamma-ray bursts; these populations are nearly disjoint. Quiescent counterparts of gamma-ray bursts are predicted to be undetectably faint.

Katz, J. I.↗
NASA NTRS1992

Detection of high-energy gamma-ray emission from the BL Lacertae object Markarian 421 by the Egret telescope on the Compton Observatory

Gamma radiation in the energy range from 50 MeV to well over 1 GeV has been observed from the direction of the BL Lac object Markarian 421 by the Energetic Gamma Ray Experiment Telescope on the Compton Observatory during the period 1991 June 27-July 11. The source flux is weak, but still statistically significant at the level of 10 exp -5; the integrated photon flux above 100 MeV is (1.4 +/- 0.3) x 10 exp -7/sq cm s. The differential photon energy spectrum can be represented by a power law with exponent 1.96 +/- 0.14. This is the first detection of gamma-ray emission from a BL Lac object.

Lin, Y. C.↗
NASA NTRS1991

Gamma-ray emission from young neutron stars

The emission models of Cheng et al. (1986) and Harding (1981) are employed to determine likely candidates for pulsed gamma-ray emission on the basis of recent radio data of pulsars. The recent detection of pulsed gamma rays from PSR 1951+32 lends observational support to the scenario of Cheng et al. which also suggests that PSR 1855+09 might be another excellent gamma-ray pulsar candidate. The possible contribution of young neutron stars to the diffuse high energy glow is also discussed.

Hartmann, Dieter H.↗