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NASA NTRS1981

X-ray and gamma-ray observations of a white-light flare

Hard radiations (X-ray and gamma-ray) from a major solar flare observed by HEAO-1 on July 11, 1978, are discussed. It is noted that the observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on C-12, both of which were previously observed in the flares of August 1972. The flare of July 11 is identified as a white-light flare. It therefore provides the first opportunity for a detailed examination of white-light flare theories that depend on proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV is found to be 1.00 + or - 0.29 ph/sq cm-sec; the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. It is concluded that the 'second-stage' acceleration of high-energy solar particles must commence promptly after the impulsive phase.

Hudson, H. S.↗

NASA NTRS1975

High energy gamma-ray radiation above 300 keV associated with solar activity

The present state of knowledge about the production of gamma-ray lines and continuum during the impulsive phase of solar flares is reviewed. It is noted that the only definite evidence for gamma rays associated with solar flares was obtained by OSO 7 during the events of August 4 and 7, 1972. This experimental evidence is examined, and gamma-ray line emission mechanisms are discussed, particularly positron annihilation through positronium formation. It is shown that if all the data are taken into account, neither the preflare nor the postflare accumulation model for gamma-ray emission can be ruled out.

Chupp, E. L.↗

NASA NTRS1978

Energetic particles in solar flares. Chapter 4 in the proceedings of the 2nd Skylab Workshop on Solar Flares

The recent direct observational evidence for the acceleration of particles in solar flares, i.e. radio emission, bremsstrahlung X-ray emission, gamma-ray line and continuum emission, as well as direct observations of energetic electrons and ions, are discussed and intercorrelated. At least two distinct phases of acceleration of solar particles exist that can be distinguished in terms of temporal behavior, type and energy of particles accelerated and the acceleration mechanism. Bulk energization seems the likely acceleration mechanism for the first phase while Fermi mechanism is a viable candidate for the second one.

Ramaty, R.↗

NASA NTRS1975

Time dependences of the 0.51 and 2.2 MeV lines in solar flares

The time dependence of the 0.51 MeV line in solar flares is determined by the following factors: variation with time of the rate of nuclear reactions which produce the positron emitters; mean lives of the positron emitters; slowing-down of the positrons which depends on the ambient density and magnetic field; and annihilation or positronium formation times of the positrons with free or bound electrons. The discussion is limited to the observed intensity-time profile of a 0.35-8 MeV gamma-ray emission and of a 0.51-MeV line calculated for three different ambient densities in the annihilation region. Major conclusions are that (1) MeV electrons and protons of at least 30 MeV/nuc have similar time dependences and hence a common origin in the flare region, which support a two-phase acceleration model in solar flares; and (2) the delayed character of the 0.51-MeV emission is clearly defined, where at a time when the rate of nuclear reactions is zero, there still is considerable 0.51-MeV emission.

Ramaty, R.↗

NASA NTRS1976

Gamma-ray astronomy: Nuclear transition region

This monograph reviews the major theoretical and experimental efforts made during the past 12 years in gamma-ray astronomy over the energy range from 10 keV to about 100 MeV, where nuclear-transition lines are expected. Early attempts to detect celestial gamma rays are recounted, mechanisms of gamma-ray line and continuum production are examined, and formulas giving the various possible differential gamma-ray spectral shapes are provided. Predicted fluxes are discussed for solar gamma rays as well as for gamma emission from supernova remnants, supernovae, neutron stars, flare stars, the galactic core and disk, black holes, and diffuse sources. Gamma-ray interactions with matter are analyzed, particularly the photoelectric effect, Compton scattering from free electrons, and pair production in nuclear fields. Significant results are summarized for observations of gamma rays from the sun as well as from point and extended sources within and beyond the Galaxy, including diffuse fluxes and transient gamma-ray bursts. Factors pertaining to the design of gamma-ray astronomy experiments are considered, especially detector background limitations, gamma-ray production within instruments, and present-day detection methods.

Chupp, E. L.↗

NASA NTRS1975

Interstellar reddening and gamma-ray emission as tracers of interstellar matter

It is shown that the gas to dust ratio in our galaxy is almost constant, even in regions of very different densities. The detailed comparison of the latitude distribution of the gamma-ray intensity with reddening data shows that in the solar vicinity the latter gives the best estimate of the total amount of interstellar gas. This conclusion is used to compute the nearby contributions to the gamma-rays longitude distribution observed by SAS II. Some implications for the large scale structure of the galaxy are examined.

Puget, J. L.↗

NASA NTRS1975

Galactic gamma radiation from cosmic rays concentrated in spiral arms

Extending the model proposed by Bignami et al. (1975), and by Fichtel et al. (1975a) for the production of the galactic high energy gamma ray distribution observed by SAS-2 to lower energies indicates the radiation is dominated by the bremsstrahlung emission of cosmic ray electrons traversing the interstellar gas. Although secondary electrons contribute only about 15% to the 10-30 MeV gamma ray emission in the solar vicinity, their contribution in the model is proportional to the third power of N, where N(r, gal. long., gal. lat.) is the total interstellar gas density, as compared to the square of N for the case of the primary components, and hence their relative importance increases in high density regions. Gamma-ray observations at these energies when compared to those at high energies (above 100 MeV) may provide a means for mapping the ratio of cosmic ray electrons to nucleons throughout the galaxy without the necessity of invoking models for the galactic magnetic field.

Kniffen, D. A.↗

NASA NTRS1978

Mechanisms and sites for astrophysical gamma ray line production

The production of gamma ray lines and estimates of line fluxes resulting from nuclear deexcitations, positron annihilation, and electron capture at various astrophysical sites are discussed. Supernova and nova explosions synthesize long-lived radioactive isotopes and eject them into space where they produce observable gamma ray lines by decaying into excited levels of daughter nuclei or by emitting positrons. Energetic charged particles in the interstellar medium, in supernova remants, in solar or stellar flares, and possibly in the vicinity of compact objects, produce gamma-ray lines by inelastic collisions which either excite nuclear levels or produce positrons and neutrons. Energetic particles can result from acceleration in time-varying magnetic fields (solar flares) or from gravitational accretion onto neutron stars and black holes. Electromagnetic processes in the strong magnetic fields of pulsars can produce positron-electron pairs, with line emission resulting from positron annihilation. Deexcitations of quantized states in strong magnetic fields can also produce lines.

Ramaty, R.↗

NASA NTRS1977

Gamma-rays from the de-excitation of 12C(15.11 MeV) and 12C(4.44 MeV) as probes of energetic particle spectra

The flux of 15.11 MeV gamma-rays relative to the flux 4.44 MeV gamma-rays was calculated from measured cross sections for excitation of the corresponding states of 12C and from experimental determinations of the branching ratios for direct de-excitation of these states to the ground state. Because of the difference in threshold energies for excitation of these two levels, the relative intensities in the two lines are particularly sensitive to the spectral distribution of energetic particles which excite the corresponding nuclear levels. For both solar and cosmic emission, the observability of the 15.11 MeV line is expected to be enhanced by low-source background continuum in this energy range.

Crannell, C. J.↗

NASA NTRS1977

Gamma-ray astrophysics

The experimental development of gamma-ray astronomy and detectors for gamma-ray observations is reviewed together with recent results obtained by the SAS-2, COS-B, and other satellites. Diffuse gamma-ray emission from the galactic plane is discussed, various localized sources are examined, and the primary production processes for celestial gamma-rays are treated in relation to the difficulty involved in detecting this radiation. Several different types of gamma-ray detectors are described in detail, and the production of high-energy gamma-rays by cosmic-ray interactions with interstellar matter is investigated. Observations of gamma-rays from the Crab Nebula and pulsar, the Vela pulsar, Cen A, and other point or localized sources are summarized with observations of nuclear gamma-ray lines during solar flares. Possible interpretations are considered for the diffuse gamma-radiation detected in regions away from the galactic plane.

Fichtel, C. E.↗

NASA NTRS1980

Gamma-ray burst observations from the UCB/LASL experiment of ISEE-3

The University of California at Berkeley (UCB)/Los Alamos Scientific Laboratory (LASL) gamma-ray burst experiment on board ISEE-3 is described and initial results of the experiment are presented. The instrument consists of an X-ray spectrometer originally designed to monitor solar emission continuously over the energy range 4.8-1264 keV with additional electronics to record data from the intense, short-lived gamma ray bursts at a very high rate and with good timing accuracy. Since the launch of ISEE-3 into a heliocentric orbit at the inner Lagrangian point in August, 1978, the UCB/LASL experiment has detected and recorded 12 gamma-ray bursts, including the unusually intense burst of November 4, 1978, which exhibited most of the classical gamma-ray burst characteristics, the intense burst of November 19, 1978, with an atypical time history and an unusually hard spectrum, and the extended burst of March 7, 1979. The spectral and temporal data demonstrate the diverse nature of the gamma-ray burst phenomenon, and are currently being combined with those of the Pioneer Venus Orbiter and Venera 11 and 12 to obtain accurate burst locations.

Evans, W. D.↗

NASA NTRS1980

HEAO 1 observations of gamma-ray lines from a solar flare

HEAO 1 observed gamma radiation, including the 2.223 MeV line of deuterium and the 4.43 MeV line of C-12, from a white-light solar flare of 1978 July 11. Line strengths over a 4 minute integration were 1.00 + or - 0.29 and 0.18 + or - 0.07 photons/sq cm s, respectively, and the continuum in the 1-5 MeV range fitted a spectrum 10 E to the -3rd photons/sq cm s keV. The 2.2 MeV line lagged 94 + or - 30 s behind the gamma-ray continuum, which itself was delayed about 20 s from the hard X-ray (not less than 40 keV) and microwave fluxes. This is the second flare for which both MeV-range lines and continuum have been observed, and the first for which simultaneous white-light observations exist. The prompt gamma-ray line (4.43 MeV) can be directly interpreted as an energy deposition of not greater than 7 x 10 to the 26th ergs per sec by energetic protons at photospheric depths. This is insufficient to maintain the white-light continuum by normal photospheric emission mechanisms.

Hudson, H. S.↗

NASA NTRS1973

Preliminary results from the first satellite of a high-resolution germanium gamma-ray spectrometer: Description of instrument, some activation lines encountered, and studies of the diffuse spectra

Gamma radiation from terrestrial and extraterrestrial sources were investigated with a high resolution Ge(Li) spectrometer-cryogen system flown onboard a low altitude, spin stabilized, polar orbiting satellite. A brief description is given of the instrument and preliminary results obtained from earth orbit are discussed. Attempts were made to use angular distributions and geomagnetic latitude spectral variations to determine diffuse background spectrum, detect gamma ray line emissions from solar flares, and search for positron annihilation radiation coming from the direction of the galactic center.

Nakano, G. H.↗

NASA NTRS1974

Search for optical emission from cosmic gamma-ray bursts

Long-exposure films are available for both times and regions of sky for several cosmic gamma-ray bursts. A visual magnitude of no more than 6 was set for optical emission from the well-observed May 14, 1972, event. The corresponding lower limit for the ratio of X-ray to optical energy flux in the burst is greater than 100. This may restrict solar-flare analogy models for the burst sources.

Grindlay, J. E.↗

NASA NTRS1975

Impulsive solar flare X-rays greater than 10 keV and some characteristics of cosmic gamma-ray bursts

Observations of impulsive solar flare X-rays greater than 10 keV are summarized and their interpretation in terms of nonthermal and thermal electron spectra is discussed. This is followed by a brief consideration of models of the hard X-ray source and the requirements of the electron acceleration process during the flash phase of solar flares. Finally, the characteristics of the recently discovered cosmic gamma-ray bursts are compared with those of the impulsive solar X-ray bursts. If both types of emissions are interpreted as bremsstrahlung from energetic electrons, then the electron spectra must be widely different in the two cases. For example, in case of solar flares, most of the energy is carried by electrons with energies of about 5 keV. On the other hand, electrons with kinetic energy of about 300 keV carry most of the energy in the cosmic source.

Kane, S. R.↗

NASA NTRS1976

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, the electron spectrum and the physical properties of the common emitting region of these radiations were evaluated. Information was also obtained on energetic protons in this flare by using gamma ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma ray continuum, it was concluded that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates approximately less than 100 keV electrons in flares.

Bai, T.↗

NASA NTRS1977

The galactic distribution (in radius and Z) of interstellar molecular hydrogen

Observations of the galactic longitude and latitude distributions of lambda = 2.6 mm CO emission are presented. Analysis of these spectral-line data yields the large-scale distribution of molecular clouds in the galactic disk and their z-distribution out of the disk. Strong maxima in the number of molecular clouds occur in the galactic nucleus and at galactic radii 4 to 8 kpc. The peak at 4 to 8 kpc correlates well with a region of enhanced 100-MeV gamma-ray emissivity. This correlation strongly supports the conclusion that the gamma-rays are produced as a result of cosmic ray interactions in molecular H2 clouds rather than in H(I). The width of the cloud layer perpendicular to the galactic plane between half-density points is 105 plus or minus 15 pc near the 5.5-kpc peak. The total mass of molecular gas in the interior of the galaxy exceeds that of atomic hydrogen and is 30 to the 9th power solar mass based on these observations.

Scoville, N. Z.↗

NASA NTRS1977

Studies on solar hard X-Rays and gamma-rays: Compton backscatter, anisotropy, polarization and evidence for two phases of acceleration

Observations of solar X-rays and gamma-rays from large flares show that the hard X-ray spectrum extends into the gamma ray region, where a flattening in the spectrum of the continuum emission is observed above about 1 MeV. This emission is believed to be due to bremsstrahlung. In addition to electron-proton collisions, at energies greater than approximately 500 keV, bremsstrahlung due to electron-electron collisions becomes significant. Bremsstrahlung production was calculated for a variety of electron spectra extending from the nonrelativistic region to relativistic energies and electron-electron bremsstrahlung is taken into account. By comparing these calculations with data, it is shown that the flattening in the spectrum of the continuum emission can be best explained by an electron spectrum consisting of two distinctive components. This evidence, together with information on the X-ray and gamma ray time profiles, implied the existence of two phases of acceleration. The first phase accelerates electrons mainly up to about several hundred keV; the second phase accelerates a small fraction of the electrons accelerated in the first phase to relativistic energies and accelerates protons to tens and hundreds of MeV.

Bai, T.↗