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NASA NTRS1980

Pair annihilation in superstrong magnetic fields

The kinematical and dynamical aspects of the annihilation processes in superstrong magnetic fields are studied. The feasibility and potential significance of detecting from magnetic neutron stars are discussed. The discussion proceeds from the derivation of the fully relativistic differential cross sections and annihilation rates for both one- and two-photon emission from a ground-state gas of electrons and positrons in a static, uniform magnetic field.

Daugherty, J. K.↗
NASA NTRS1981

Synchrotron cooling and annihilation of an e/+/-e/-/ plasma - The radiation mechanism for the 5 March, 1979 transient

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the 5 March, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation-dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. In this layer, about 10 to the 12th generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the 0.15 s duration of the impulsive phase.

Ramaty, R.↗
NASA NTRS1981

Variable positron annihilation radiation from the galactic center region

HEAO 3 Cosmic Gamma-Ray Spectrometer evidence is presented for the existence of a time-varying, unshifted, narrow 511 keV line emission from the vicinity of the galactic center. Although uncertainties exist regarding the spatial extent of the features as well as its centroid, all data are consistent with emission from a single point source located at the galactic center. This interpretation would require a source luminosity of 2 x 10 to the 37th ergs/sec, and a positron annihilation rate of about 10 to the 43rd/sec. It is concluded that a variable source of positrons which could generate such an annihilation figure might be a massive black hole at the galactic center, as has been suggested by IR observations.

Riegler, G. R.↗
NASA NTRS1981

HEAO 3 measurements of the atmospheric positron annihilation line

The atmospheric positron annihilation line at 511 keV was the first gamma-ray line observed which originates from processes of astrophysical importance. Measurements of the properties of a positron annihilation line make it possible to determine many of the physical conditions present at the source of emission. An investigation was conducted of the 511-keV line measured with the HEAO C-1 experiment. The HEAO C-1 experiment consisted of a cluster of four, large, high-purity germanium detectors surrounded by an active anticoincidence CsI shield. A strong atmospheric line was measured at 511.07 + or - 0.10 keV. The net atmospheric emission has been accurately determined as a function of geomagnetic latitude and varies from 0.01 photons/sq cm-sec-sr near the magnetic equator to 0.035 photons/sq cm-sec-sr at a latitude of 55 deg.

Mahoney, W. A.↗
NASA NTRS1982

Moisture dependence of positron annihilation rates in molecular substances

Positron annihilation rates have been studied in polymers and graphite-polymer composites as a function of their moisture content. The annihilation rates have been found to increase linearly with increasing moisture content in epoxies and polyamides, whereas no definite trends have been observed in polyimides. These experimental results have been used as the basis for the calculation of moisture content of several polymeric test specimens. For example, the directly measured moisture content of a Kevlar specimen was 45.5 + or - 5.0% of saturation value, whereas the moisture content on the basis of the decrease in positron lifetime was calculated to be 46.5 + or - 3.5%. Similarly, the directly measured moisture content of a graphite-epoxy composite (55 v/o fiber) was 19.2 + or - 0.6% of saturation value as opposed to a calculated value of 16.0 + or - 5.0%.

Singh, J. J.↗
NASA NTRS1981

Positron annihilation radiation from the galactic center region

Measurements of 0.511 MeV radiation from the vicinity of the galactic center performed by the high resolution gamma ray spectrometer aboard the HEAO 3 spacecraft are discussed. The spectrometer is described and an exploded view of it exhibited, and scans in the 50-244 keV region as well as 0.511 MeV are shown. The intensity from the Crab nebula remained constant while that of the Cygnus source increased considerably, and there was a decrease in the radiation from the vicinity of the galactic center. The flux emanating from the center implies an annihilation rate of 10 to the 43rd per second. The apparent time variation and the spatial extent observed tend to limit the source to a single object and the annihilation region to a size less than about one light year.

Jacobson, A. S.↗
NASA NTRS1983

Upper Limits to the Annihilation Radiation Luminosity of Centaurus a

A high resolution observation of the active nucleus galaxy Centaurus A (NGC 5128) was made by the GSFC low energy gamma-ray spectrometer (LEGS) during a balloon flight on 1981 November 19. The measured spectrum between 70 and 500 keV is well represented by a power law of the form 1.05 x 10 (-4) (E/100 keV) (-1.59) ph/sq cm /s with no breaks or line features observed. The 98% confidence (2 sigma) flux upper limit for a narrow ( 3 keV) 511-keV positron annihilation line is 9.9 x 10 (-4) ph/ sq cm /s. Using this upper limit, the ratio of the narrow-line annihilation radiation luminosity to the integral or = 511 keV luminosity is estimated to be 0.09 (2 sigma upper limit). This is compared with the measured value for our galactic center in the Fall of 1979 of 0.10 to 0.13, indicating a difference in he emission regions in the nuclei of the two galaxies.

Gehrels, N.↗
NASA NTRS1983

Electron-positron Annihilation and the Cosmic X-ray Background

The possibility that the processes responsible for the cosmic X-ray background (CXB) would also produce an e(-)-e(+) annihilation feature is examined. Under the assumption that these processes are thermal, the absence of a strong e(-)-e(=) annihilation feature places constraints on the compactness (L/R ratio) of these sources. Observations favor souces of small compactness ratio.

Kazanas, D.↗
NASA NTRS1983

The Origin of the Galactic Annihilation Radiation

Observations of the e(+)-e(-) annihilation radiation from the galactic center are reviewed and the implications of these and other observations on the positron production process, the annihilation region and the fundamental nature of the galactic center source are discussed.

Lingenfelter, R. E.↗
NASA NTRS1983

The e(+)-e(-) annihilation line and the cosmic X-ray background

The possibility that the processes responsible for the cosmic X ray background (CXB) would also produce an a positron-electron annihilation feature is examined. Under the assumption that these processes are thermal, the absence of a strong electron-positron annihilation feature places constraints on the compactness (L/R ratio) of these sources. Observations favor sources of small compactness ratio.

Kazanas, D.↗
NASA NTRS1984

Search for 511 keV electron-positron annihilation radiation from mildly active galaxies using the HEAO 3 gamma-ray spectrometer

The HEAO 3 spacecraft's gamma-ray spectrometer has been used to obtain upper limits to the 511-keV electron-positron annihilation line flux from seven galaxies: M 81/M 82, NGC 4278, M 104, NGC 6500, NGC 2911, and NGC 262. These appear to contain scaled-up versions of the radio source in the galactic center, and their nonthermal activity appears to be confined to the central regions, implying that energetic particles produced in these nuclei do not escape into the low density regions of interstellar or intergalactic space. On the basis of the scaling law adopted, six of these seven galaxies should have been detected above the upper limits obtained for narrow 511 keV emission. Explanations are adduced which account for the scaling law's failure to predict the correct level of radiation annihilation.

Marscher, A. P.↗
NASA NTRS1985

Diffuse galactic annihilation radiation from supernova nucleosynthesis

The propagation of MeV positrons in the outer ejecta of type I supernovae was investigated. It was found that the positrons created at times of approx 100 days propagated along magnetic field lines in the outer ejecta without any appreciable pitch-angle scattering or excitation of hydromagnetic waves. The lack of significant pitch-angle scattering is well consistent with models of wave excitation and scattering by resonant interactions. This occurs because time periods to scatter the particles or to excite waves are significantly longer than escape times. Thus it is expected that, when positrons are not coupled to the ejecta by Coulomb collisions, they escape from the relatively cold, dense ejecta and reside predominantly in the tenuous, hotter, shock-heated interstellar gas. In the tenuous shock-heated gas the positron lifetime against annihilation is much greater than lifetimes in the dense ejectra. Thus the production of steady-state diffuse annihilation radiation by some fraction of these escaped positrons seems probable.

Higdon, J. C.↗
NASA NTRS1983

Positron annihilation radiation from solar flares

Positron-annihilation radiation has been observed from the June 21, 1980 and June 3, 1982 flares by the gamma-ray spectrometer on the Solar Maximum Mission satellite. The observed 0.511-MeV line fluences from the flares were 14.6 + or - 3.3 gamma/sq cm and 103 + or - 8 gamma/sq cm, respectively. Measurement of the line width establishes an upper limit to the temperature in the annihilation region of 3 x 10 to the 6th K. The time dependence of the 0.511-MeV line during the 1980 flare is consistent with the calculations of Ramaty et al. (1983) for positrons created in the decay of radioactive nuclei. The time dependence of the 0.511-MeV line for the 1982 flare is more complex and requires more detailed study.

Share, G. H.↗
NASA NTRS1983

The origin of the Galactic center annihilation radiation

Observations of the positron-electron annihilation radiation from the Galatic Center suggest that something truly extraordinary is occurring there. The observations of the intense, time-varying, 0.511-MeV emission are reviewed, and the implications of these and other recent observations on the positron production process, the annihilation region and the fundamental nature of the Galactic Center Source are discussed.

Lingenfelter, R. E.↗
NASA NTRS1983

Upper limits to the annihilation radiation luminosity of Centaurus A

A high resolution observation of the active nucleus galaxy Centaurus A (NGC 5128) was made by the GSFC low energy gamma-ray spectrometer (LEGS) during a balloon flight on 1981 November 19. The measured spectrum between 70 and 500 keV is well represented by a power law of the form 1.05 x 10 (-4) (E/100 keV) (-1.59) ph/sq cm/s with no breaks or line features observed. The 98 percent confidence (2 sigma) flux upper limit for a narrow (3 keV) 511-keV positron annihilation line is 9.9 x 10 (-4) ph/sq cm/s. Using this upper limit, the ratio of the narrow-line annihilation radiation luminosity to the integral or = 511 keV luminosity is estimated to be 0.09 (2 sigma upper limit). This is compared with the measured value for our Galactic center in the Fall of 1979 of 0.10 to 0.13, indicating a difference in the emission regions in the nuclei of the two galaxies.

Gehrels, N.↗
NASA NTRS1988

Cascade model of gamma-ray bursts: Power-law and annihilation-line components

If, in a neutron star magnetosphere, an electron is accelerated to an energy of 10 to the 11th or 12th power eV by an electric field parallel to the magnetic field, motion of the electron along the curved field line leads to a cascade of gamma rays and electron-positron pairs. This process is believed to occur in radio pulsars and gamma ray burst sources. Results are presented from numerical simulations of the radiation and photon annihilation pair production processes, using a computer code previously developed for the study of radio pulsars. A range of values of initial energy of a primary electron was considered along with initial injection position, and magnetic dipole moment of the neutron star. The resulting spectra was found to exhibit complex forms that are typically power law over a substantial range of photon energy, and typically include a dip in the spectrum near the electron gyro-frequency at the injection point. The results of a number of models are compared with data for the 5 Mar., 1979 gamma ray burst. A good fit was found to the gamma ray part of the spectrum, including the equivalent width of the annihilation line.

Harding, A. K.↗
NASA NTRS1987

Positron annihilation at the Galactic center

The efficiency of production of positrons by gamma-gamma interaction under conditions which may occur in the Galactic center. Their annihilation line and three-photon annihilation continuum contribute to the emitted radiation. If a geometrical model is assumed, it will be possible to determine the parameters of the gamma-ray source region. By comparing the computed spectra to observations, a rough estimate is made using extant data.

Carrigan, B. J.↗
NASA NTRS1988

Cosmic-ray antiprotons, positrons, and gamma rays from halo dark matter annihilation

The subject of cosmic ray antiproton production is reexamined by considering other choices for the nature of the Majorana fermion chi other than the photino considered in a previous article. The calculations are extended to include cosmic-ray positrons and cosmic gamma rays as annihilation products. Taking chi to be a generic higgsino or simply a heavy Majorana neutrino with standard couplings to the Z-zero boson allows the previous interpretation of the cosmic antiproton data to be maintained. In this case also, the annihilation cross section can be calculated independently of unknown particle physics parameters. Whereas the relic density of photinos with the choice of parameters in the previous paper turned out to be only a few percent of the closure density, the corresponding value for Omega in the generic higgsino or Majorana case is about 0.2, in excellent agreement with the value associated with galaxies and one which is sufficient to give the halo mass.

Rudaz, S.↗