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NASA NTRS1984

Al-26 in the interstellar medium

The amount of dispersed interstellar Al-26 detected by the HEAO 3 gamma-ray spectrometer cannot have been synthesized by supernova explosions if current calculations of the production ratio p(26)/p(27) approximately equal to 0.001 are correct. Simple models of chemical evolution of the Galaxy are presented to explain this point. The observed Al-26 is more likely due to about 100 million dispersed novae, or to a single old (10,000-1,000,000 yr) supernova remnant that today surrounds the solar system. If the Al-26 is dispersed, the high interstellar ratio today Al-26/Al-27 about equal to 0.00002 calls into question the requirement that a supernova trigger for formation of the solar system was the cause of a concentration 3-times larger. Also discussed is p-process production in novae with application to the question of live Sm-146 in the solar system.

Clayton, D. D.↗

NASA NTRS1984

Observation of stellar remnants from recent supernovae

The current observational situation regarding the presence or absence of collapsed stellar objects associated with supernova remnants is reviewed. The theoretical expectations and observational evidence for the various possible classes of stellar remnants are discussed. The results of searches for these objects in the radio, optical, X-ray and gamma-ray regions of the spectrum are outlined with the conclusion that nine bona fide candidates are currently known. The implications of these results for the origin and evolution of compact objects are summarized.

Helfand, D. J.↗

NASA NTRS1984

Gamma rays of 0.3-30 MeV from PSR0833 - 45

Pulsed gamma rays from the Vela pulsar PSR0833 - 45 are reported for the first time over the medium energy range 0.3-30 MeV. They were observed with a double scatter gamma-ray telescope flown on a balloon over Australia on Nov. 10, 1981, 31 days after a large glitch in the pulsar period. The first and second pulses were detected from single scatters in the top scintillators at energies over 0.3 MeV with statistical significances of 1.6 sigma each, and by double scatters from all detector cell pairs at energies of 1-30 MeV with significance of 4.8 sigma and 3.8 sigma. The phase separation of the two pulses is 0.43 + or - 0.02 at the same absolute phases previously found by SAS2 and COS B for energies over 35 MeV.

Tumer, O. T.↗

NASA NTRS1983

A mercuric iodide detector system for X-ray astronomy. II - Results from flight tests of a balloon borne instrument

To establish the expected sensitivity of a new hard X-ray telescope design, described by Ricker et al., an experiment was conducted to measure the background counting rate at balloon altitudes (40 km) of mercuric iodide, a room temperature solid state X-ray detector. The prototype detector consisted of two thin mercuric iodide (HgI2) detectors surrounded by a large bismuth germanate scintillator operated in anticoincidence. The bismuth germanate shield vetoed most of the background counting rate induced by atmospheric gamma-rays, neutrons and cosmic rays. A balloon-borne gondola containing a prototype detector assembly was designed, constructed and flown twice in the spring of 1982 from Palestine, TX. The second flight of this instrument established a differential background counting rate of 4.2 + or - 0.7 x 10 to the -5th counts/s sq cm keV over the energy range of 40-80 keV. This measurement was within 50 percent of the predicted value. The measured rate is about 5 times lower than previously achieved in shielded NaI/CsI or Ge systems operating in the same energy range.

Vallerga, J. V.↗

NASA NTRS1984

Magnetohydrodynamic model of Crab nebula radiation

The properties of the synchrotron continuum are calculated on the basis of a steady spherically symmetric MHD model of the flow in the Crab nebula. The model accounts for the spectral distribution of the continuum from the optical to gamma-rays, and semiquantitatively for its observed spatial distribution. The model does not account for the radio portion of the Crab nebula's spectrum.

Kennel, C. F.↗

NASA NTRS1983

NASA astrophysics - Optical systems to explore the universe

Major and minor NASA astrophysical research efforts in the near-term are outlined, together with projections of direction for future projects. The Space Telescope is being readied for a 1986 launch and will feature an f/24, 2.4 m aperture, an MgF2 mirror with better than 1/60 wavelength accuracy and will be diffraction-limited in the UV. Pointing accuracy is designed to be 0.007 arcsec for 24 hr. Optical, spectrometric, and photometric equipment will be included. Around 1990, Shuttle-based missions will include an IR telescope and a subarcsec solar surface imaging device. A free-flying X-ray observatory (AXAF) is planned and will include a sensitivity that exceeds that of the HEAO-2 spacecraft by two orders of magnitude. Instruments are under development for higher resolution UV, gamma-ray, and IR studies. In-orbit interferometry is being studied and will depend on in-orbit assembly and servicing of stable structures with segmented optics.

Pellerin, C. J., Jr.↗

NASA NTRS1983

Diffuse galactic emission of high energy gamma rays

A study of the gamma-radiation produced by the interaction of cosmic rays with the interstellar matter and photons is extended, based on recent COS-B satellite results and background estimates (1982) and evaluations of the 21-cm radiation in the Galaxy. Gamma-ray intensities predicted over the latitude range of -10 to 10 degrees for energy ranges 70-150, 150-300, and 300-5000 MeV agree well with the COS-B longitude distributions. The agreement suggests that the radial distribution of the emission, calculated from the model, is also approximately correct. The energy spectrum is calculated using COS-B and SAS-2 data. Finally, it is noted that contribution by the arms on the far side of the Galaxy for small galactic latitudes (between -0.4 and +0.4 degrees) makes it possible to identify very large far-side molecular clouds.

Kniffen, D. A.↗

NASA NTRS1983

Plausible mechanisms for rapid acceleration of protons during solar flares

The distinctive physical features of 20 gamma-ray line flares observed during Solar Maximum Mission are discussed. The average energy deposition by nonthermal electrons in the impulsive phase of the flare event of April 27, 1980 is calculated, on the basis of spectral data from the Hinotori satellite. Analysis of the hard X-ray spectral evolution of the flare events of June 21, 1980 and April 27, 1981, showed a very hot (about 10 to the 8th K) thermal component with an emission measure of 3 x 10 to the 47th per cu cm. Some possible mechanisms for the particle acceleration observed during the flare events are discussed, including: the delay of high-energy hard X-rays; first-order Fermi acceleration by shocks propagating in a closed flare loop; and the compression of accelerated particles by upstream and downstream scattering centers around a shock front. In addition to the above mechanisms, sotchastic acceleration by turbulence is identified as a possible mechanism for rapid proton acceleration during solar flare events.

Bai, T.↗

NASA NTRS1983

A high energy resolution experiment in the hard X-ray range

The Low Energy Gamma-Ray Spectrometer (LEGS), a joint project among NASA/GSFC, CEN-SACLAY, and Rice University, is designed to perform fine energy resolution measurements of astrophysical sources. In its low energy configuration (20-800 keV energy range) the instrument uses three planar detectors (effective area of 53 sq cm) surrounded by a combination of passive Fe and active NaI for shielding and collimation (FOW of about 5 x 10 deg FWHM). In a typical one-day balloon flight, LEGS sensitivity limit (3 sigma) for narrow line features is not greater than about 0.0003 ph/sq cm s (at 100 kev).

Paciesas, W.↗

NASA NTRS1983

High sensitivity high resolution double scatter 1-30 MeV gamma ray telescope

The design and performance of the double-scatter 1-30-MeV gamma-ray telescope developed at the University of California for balloon-borne observations are reported and illustrated with drawings, diagrams, and graphs. The telescope comprises an upper array of 16 6.03 x 6.03 x 100-cm plastic scintillator bars viewed by photomultiplier tubes (PMTs) at both ends, a lower array (at 100-cm distance) of 16 4.83 x 4.83 x 100-cm NaI(Tl) bars viewed at both ends by PMTs, and four 122 x 122 x 0.64-cm plastic sheets viewed by four PMTs each as charged-particle anticoincidence shielding. The operating principle of the telescope, the tests conducted on the NaI(Tl) bars, the on-board data-processing electronics, and the overall performance are characterized. Performance parameters include area 1 sq m, field of view pi sr, 4-sigma sensitivity above 1 MeV 0.00004 gamma/sq cm sec over 25 h, angular resolution about 3.5 deg FWHM, and energy resolution 10 percent at 1 MeV and 6 percent at 6 MeV.

Zych, A. D.↗

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

Pair production near threshold in pulsar magnetic fields

In Pulsar polar cap models, curvature radiation gamma-rays produce e(+) e(-) pairs in the strong magnetic fields near the surface of the neutron star. While these gamma rays havea energies E sub gamma mc(2), they also propagate at very small angies to the fields, such that the threshold condition, E gamma 2mc(2)/sin theta is just barely satisfied when they pair produce. Threshold effects on the pair production attenuation coefficient, which are due to the discreteness of the e(+) e(-) Landau states, must therefore be considered when computing the mean free paths of curvature radiation photons in pulsar magnetic fields. These effects, which are not incorporated in the asymptotic expression for the attenuation coefficient, have some interesting consequences for pulsar models. Since pair production is suppressed near threshold, the photon mean free paths are longer than previously thought. In magnetic fields greater than approximately 6 x 10 to the 12th G, the pairs tend to be produced in the ground state Landau level and will not synchrotron radiate. Since synchrotron radiation is an essential ingredient in the eiectromagnetic cascades which produce low energy pairs above the acceleration region, pulsars with very high magnetic fields may not produce many pairs.

Harding, A. K.↗

NASA NTRS1983

Low-temperature positron annihilation

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the Galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.↗

NASA NTRS1983

Are mildly active galaxies sources of electron-positron annihilation radiation?

Mildly active galaxies are examined as possible sources of 511-keV line radiation resulting from electron-positron annihilations. Observations of seven galaxies of various types are made using an HEAO-3 gamma-ray spectrometer. It is shown that none of the galaxies are detected in the 511 keV line and therefore the proposed scaling law for 511-keV line emission from the galactic nuclei is not valid. The results indicate that the scaling overestimates the 511-keV line flux by a factor of 3 or more. Possible causes of this discrepancy are considered. The expected values of and the observed 2.3-sigma upper limits to the 511-keV line flux for each source and the Galactic Center are presented in a table.

Marscher, A. P.↗

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 NTRS1983

Comparison of photon-photon and photon-magnetic field pair production rates

Neutron stars were proposed as the site of gamma-ray burst activity and the copious supply of MeV photons admits the possibility of electron-positron pair production. If the neutron star magnetic field is sufficiently intense (10 to the 12th power G), both photon-photon (2 gamma) and photon-magnetic field (gamma) pair production should be important mechanisms. Rates for the two processes were calculated using a Maxwellian distribution for the photons. The ratio of 1 gamma to 2 gamma pair production rates was obtained as a function of photon temperature and magnetic field strength.

Burns, M. L.↗

NASA NTRS1988

Supernova 1987A

Supernova 1987A (February 23, 1987) in the Large Magellanic Cloud is the brightest supernova to be observed since SN 1604 AD (Kepler). Detection of a burst of neutrinos indicates that a neutron star was formed. Radioactive decay of about 0.07 solar mass of Co-56 is responsible for the observed optical light as well as hard X-rays and gamma-ray lines. Ultraviolet, optical, and infrared 'light echoes' and soft X-rays provide information on the distribution of circumstellar matter and the evolution of the progenitor star.

Mccray, Richard↗

NASA NTRS1971

Neutron Activation Analysis of Water - A Review

Recent developments in this field are emphasized. After a brief review of basic principles, topics discussed include sources of neutrons, pre-irradiation physical and chemical treatment of samples, neutron capture and gamma-ray analysis, and selected applications. Applications of neutron activation analysis of water have increased rapidly within the last few years and may be expected to increase in the future.

Buchanan, John D.↗