Directionality of Solar Flare Accelerated Protons and Alpha Particles from Gamma-Ray Line Measurements

The energies and widths of gamma-ray lines emitted by ambient nuclei excited by flare-accelerated protons and alpha particles provide information on the ions directionality and spectra, and on the characteristics of the interaction region. We have measured the energies and widths of strong lines from de-excitations of 12C, 16O, and 20Ne in solar flares as a function of heliocentric angle. The line energies from all three nuclei exhibit ~1% redshifts for flares at small heliocentric angles, but are not shifted near the limb. The lines have widths of ~3% FWHM. We compare the 12C line measurements for flares at five different heliocentric angles with calculations for different interacting-particle distributions. A downward isotropic distribution (or one with a small upward component) provides a good fit to the line measurements. An angular distribution derived for particles that undergo significant pitch angle scattering by MHD turbulence in coronal magnetic loops provides comparably good fits

The Gamma-Ray Light Curves of SN 1987A

Observations of the SN 1987A ejecta in four Co-56-decay gamma-ray lines, obtained using the SMM gamma-ray spectrometer between February 1987 and May 1989, are reported and analyzed. The instrument characteristics and data-reduction procedures are described, and the results are presented in extensive tables and graphs and discussed with reference to theoretical models. Gamma-ray fluxes significantly above possible instrumental levels (as determined from analysis of pre-1987 data) were detected in the second half of 1987 and the first half of 1988. The data are found to favor a model with some Co-56 in regions of low gamma-ray optical depth by 200 d after the SN outburst over models with all Co-56 at one depth within a uniform expanding envelope. Also investigated are the gamma-ray contribution to the total bolometric luminosity and the escape (and potential observability) of Co-57 gamma rays

Gamma-Ray Limits on Galactic Fe-60 and Ti-44 Nucleosynthesis

We have searched nearly 10 years of data from NASA\u27s Solar Maximum Mission (SMM) Gamma-Ray Spectrometer for evidence of gamma-ray line emission from the decay of the shorter lived daughters, Co-60 and Sc-44, of nucleosynthetic Fe-60 and Ti-44. The data are compared with models of the expected signals from the annual scan of the ecliptic by SMM. These models include (1) the extended diffuse emision from the many supernovae which should contribute Fe-60 over its 2.2 Myr lifetime, and (2) point sources at various locations in the Galactic plane which could be previously undiscovered remnants of supernovae which ejected Ti-44. We find no evidence of Galactic emission from either nucleus; upper limits (99% confidence) are near 8 x 10-5 photons/sq cm/s, for both the 1.17 MeV line from Co-60 decay integrated over the central radian of Galactic longitude and for the 1.16 MeV line from Sc-44 from points near the Galactic center. The limits on 1.16 MeV flux from longitudes near + or - 90 deg rise to approximately 2 x 10-4 photons/sq cm/s because the large angular distance to the ecliptic reduces the sensitivity in those directions. The mass of Fe-60 in the interstellar medium today is constrained to be less than 1.7 solar mass. This sets a limit on the current Galactic production rate of Fe-60 and of other isotopes coproduced with it, for example, Ca-48 and Ti-50. Estimating the current production of these stable isotopes from their solar abundances suggests that there should be about 0.9 solar mass of Fe-60 in the interstellar medium and indicates that Fe-60 could soon be detected with a slightly more sensitive instrument. Comparing the estimated production rates of stable isotopes with the gamma-ray limits on those of radioactive isotopes allows us to constrain some models of Galactic chemical evolution. The mass of Ti-44 at the Galactic center, for example, is (99% confidence) less than 8 x 10-5 solar mass. This is a quite improbable result viewed in either of two ways. Employing plausible models of Galactic chemical evolution constrained to produce the solar concentration of Ca-44 in the Galaxy 4.5 Gyr ago suggests that Ca-44 is produced today at the rate (3-4) x 10-4 solar mass per century. This production rate is consistent with our measurement at 5% confidence only for supernova rates less than 1.5 per century, depending slightly on the actual Ti-44 lifetime, and assuming all Ca-44 is ejected as Ti-44. Lower rates are consistent with our data, because the implied interval with no supernovae is not so unlikely, but the required higher yields of Ti-44 begin to strain current supernova nucleosynthesis calculations. Apart from the solar abundance requirement, we can check the consistency of any combination of supernova rate and Ti-44 yield. A Galactic supernova rate of three per century and a yield 10-4 solar mass of Ti-44 per event, both very reasonable estimates, are consistent with our data at only 5% confidence. Perhaps the typical yield of frequent supernovae is significantly smaller than this, and the source of most Ca-44 is a rare type of high-yield of frequent supernovae is significantly smaller than this, and the source of most Ca-44 is a rare type of high-yield event which has not occurred recently. The isotope Ti-44 is probably not a major contributor to interstellar positions

A Search for the 478 keV Line from the Decay of Nucleosynthetic 7Be

Unstable Be-7 (half-life 53.28 days) is expected to be present in the ejecta of classical novae. If the frequency of novae in the central Galaxy is high enough, a nearly steady state abundance of Be-7 will be present there. Data accumulated during transits of the Galactic center across the aperture of the Solar Maximum Mission Gamma Ray Spectrometer have been searched for evidence of the 478 keV gamma-ray line resulting from Be-7 decay. A 3-sigma upper limit of 0.00016 gamma/sq cm s has been placed on the emission in this line from the central radian of the Galactic plane. Less stringent limits have been set on the production of Be-7 in Nova Aquilae 1982, Nova Vulpeculae 1984 No. 2, and Nova Centauri 1986 from observations with the same instrument

SMM Observations of Gamma-Ray Transients. 3: A Search for a Broadened, Redshifted Positron Annihilation Line from the Direction of the Galactic Center

We have searched for 1980-1988 Solar Maximum Mission gamma-ray spectrometer data for transient emission on timescales from hours to approximately 12 days of broad gamma-ray lines at energies approximately 400 keV, which were reported by the High Energy Astronomy Observatory (HEAO) 1 and SIGMA experiments from two sources lying toward the Galactic center. The lines have been interpreted as the product of the annihilation of positrons in pair plasmas surrounding the black hole candidate 1E 1740.7-2942 and the X-ray binary 1H 1822-371. Our results from a combined exposure of approximately 1.5 x 107s provide no convincing evidence for transient emission of this line on any timescale between approximately 9 hr and approximately 1 yr. Our 3 sigma upper limits on the line flux during approximately 12 day intervals are characteristically 4.8 x 10-3 photon/sq cm/s, while for approximately 1 day intervals our 3 sigma upper limits are characteristically 4.9 x 10-3 photon/sq cm/s. These results imply a duty cycle of less than 1.3% for the transient line measured from 1H 1822-371 during a approximately 3 week interval in 1977 by HEAO 1, and a duty cycle of less than or = 0.8% for the transient line detected in 1990 and 1992 from 1E 1740.7-2942 on approximately 1 day timescales by SIGMA

SMM observations of gamma-ray transients. 2: A search for gamma-ray lines between 400 and 600 keV from the Crab Nebula

We have search spectra obtained by the Solar Maximum Mission Gamma-Ray Spectrometer during 1981-1988 for evidence of transient gamma-ray lines from the Crab Nebula which have been reported by previous experiments at energies 400-460 keV and 539 keV. We find no evidence for significant emission in any of these lines on time scales between aproximately 1 day and approximately 1 yr. Our 3 sigma upper limits on the transient flux during 1 d intervals are approximately equal to 2.2 x 10-3 photons/sq cm/s for narrow lines at any energy, and approximately equal to 2.9 x 10-3 photons/sq cm/s for the 539 keV line if it is as broad as 42 keV Full Width at Half Maximum (FWHM). We also searched our data during the approximately 5 hr period on 1981 June 6 during which Owens, Myers, & Thompson (1985) reported a strong line at 405 keV. We detected no line down to a 3 upper sigma limit of 3.3 x 10-3 photons/sq cm/s in disagreement with the flux 7.2 +/- 2.1 x 10-3 photos/sq cm/s measured by Owens et al

Spatial and Temporal Variability of the Gamma Radiation from Earth\u27s Atmosphere During a Solar Cycle

The Solar Maximum Mission satellite’s Gamma Ray Spectrometer spent much of its 1980–1989 mission pointed at Earth, accumulating spectra of atmospheric albedo γ-rays. Its 28◦ orbit ensured that a range of geomagnetic latitudes was sampled. We measured the variation with time and cutoff rigidity of some key γ-ray lines which are diagnos-tic of the intensity of the Galactic cosmic radiation penetrating the geomagnetic cutoff and of the secondary neutrons produced in the atmosphere. We found that the inten-sities of nuclear lines at 1.6 MeV, 2.3 MeV and 4.4 MeV varied inversely with solar ac-tivity in cycles 21–22 as expected from the theory of solar modulation of cosmic rays. They were found to be strongly anticorrelated with cutoff rigidity, as expected from the theory of the cutoff, falling by a factor ∼ 3.6 between the lowest (\u3c 7 GV) and high-est (\u3e 13 GV) rigidities sampled. The solar cycle modulation was particularly marked at the lowest rigidities, reaching an amplitude of 16%

Gamma-Ray Limits on Na-22 Production in Novae

Data accumulated from 1980 to1987 by the gamma-ray spectrometer on SMM have been searched for evidence of cosmic line emission at 1.275 MeV. This emission would result from the decay of 22Na, which might be produced by classical nova outbursts. No evidence of any 1.275 MeV emission of celestial origin has been found. A limit of 3×10-6M_sun; is placed on the accumulated 22Na from many novae occurring near the Galactic center, and a limit of 7×10-7M_sun; is placed on the mass of 22Na ejected by the closest of the recent neon-rich novae. These limits, while lower than any previous ones, are not in conflict with recent theoretical predictions of the production of 22Na in novae. The product of the frequency and average initial neon abundance of novae of the neon-rich class is constrained by the Galactic center 22Na limit

Limits on a Variable Source of 511 keV Annihilation Radiation near the Galactic Center

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission satellite (SMM) has observed a strong Galactic source of 511 keV annihilation radiation from its launch in 1980 to its reentry in 1989. These observations are consistent with an extended source having an intensity of about 0.002 gamma/sq cm/s averaged over the central radian of Galactic longitude. These data are searched for evidence of the variable Galactic center source of 511 keV line radiation which was reported to have reappeared in 1988 by Leventhal et al. The SMM data are consistent with, but do not require, a compact source emitting a time-averaged flux of about 0.0004 gamma/sq cm/s during about 3 month transits in 1987 and 1988; they are inconsistent with a compact source flux in excess of 0.0008 gamma/sq cm/s for each year

Measurement of the 0.3-8.5 MeV Galactic Gamma-Ray Spectrum from the Galactic Center Direction

The low-energy gamma-ray spectrum from the direction of the Galactic center is determined using data obtained with the SMM Gamma-Ray Spectrometer. It is found that the diffuse gamma-ray spectrum from the Galactic center region can be interpreted in a straightforward way as the sum of five components of a presented equation. The components include a hard power law dominating the continuum at high energies caused principally by cosmic ray electron bremsstrahlung radiation, two narrow lines due to Al-26 decay and positron annihilation, an excess continuum component below 0.511 MeV consistent with the annihilation of positrons by formation of Ps, and a soft power law at low energies which is consistent with an extrapolation upward in energy of known hard X-ray sources in the Galactic center region