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  • 1
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    GRIS detections of the 511 keV line from the Galactic center region in 1992 (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The Gamma Ray Imaging Spectrometer (GRIS) was flown on balloons over Alice Springs, Australia on 1992 April 26 and May 7. A full Galactic center transit (about 12 hr) was achieved on both flights with the instrument working normally. The electron/positron annihilation line was detected on both flights. The line fluxes and line widths were found to be (7.7 +/- 1.2) x 10 exp -4 and (8.9 +/- 1.1) x 10 exp -4 photons/sq cm per sec and 1.3 +/- 0.7 and 3.6 +/- 1.0 keV, respectively. These results are compared to each other and earlier (1988) GRIS results to produce suggestive evidence for source variability. Near-contemporaneous OSSE/CGRO Galactic center observations indicate that the GRIS results cannot be due solely to a single point source like IE 1740.7-2942 within a few degrees of the Galactic center. The GRIS 1992 results represent the first time that successive high-resolution balloon measurements have been achieved on a time scale of days.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 405; 1; p. L25-L28.
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  • 2
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    GRIS background reduction results using isotopically enriched Ge (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The Gamma Ray Imaging Spectrometer (GRIS) was flown twice from Alice Springs, Australia, in the spring of 1992 for a total of 32 hr at float altitude. One of the seven Ge detectors was isotopically enriched (greater than 97% Ge-70). This was the first time an enriched-Ge detector was used for astrophysical observations. Because of its thick anticoincidence shield, the GRIS instrument background is dominated by internal beta-decay in the energy range of 200-1000 keV. Half of the contribution in this beta-decay 'hump' is due to neutron-activated Ge-74. In this energy range, GRIS observed a factor of 2 reduction in the background in the enriched detector, as predicted. In future instruments (e.g., INTEGRAL), with thicker anticoincidence shields and smaller apertures, the background reduction will be even larger. Three strong instrumental background lines (54, 67, and 139 keV) are also eliminated. The elimination of the first two is particularly important for cylotron line observations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 427; 1; p. 519-521
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  • 3
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    GRIS observations of positron annihilation radiation from the Galactic center (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: In the present Gamma-Ray Imaging Spectrometer (GRIS) observations, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. These data, in combination with those from other narrow-field observations in the 1980s, support the two-component model of the positron annihilation source. A strong hard-X-ray continuum was detected in the Galactic plane observation; this 'diffuse' continuum component solves the mystery as to why wide-field instruments have detected such high continuum emission from the Galactic center, in virtue of its source contributions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L13-L16
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  • 4
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    Gamma-ray lines from SN1987A (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Results of gamma-ray observations of SN1987A conducted over a period of 800 days using balloon-borne high-resolution germanium detectors are discussed. Most of the measurements show red-shifted line centroids (about 500 km/s) and broadened profiles, contradicting the standard homogeneous and spherically symmetric models, which predict blue-shifted lines and narrower line widths. To explain this apparent contradiction, it may be necessary to invoke fragmentation and nonspherical geometries.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 11; 8, 19
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  • 5
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    GRIS observations of Al-26 gamma-ray line emission from two points in the Galactic plane (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Both of the Gamma-Ray Imaging Spectrometer (GRIS) experiment's two observations of the Galactic center region, at l = zero and 335 deg respectively, detected Al-26 gamma-ray line emission. While these observations are consistent with the assumed high-energy gamma-ray distribution, they are consistent with other distributions as well. The data suggest that the Al-26 emission is distributed over Galactic longitude rather than being confined to a point source. The GRIS data also indicate that the 1809 keV line is broadened.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L9-L12
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  • 6
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    Observations of gamma-ray line profiles from SN 1987A (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Fully resolved gamma-ray line observations from the decay of Co-56 in SN 1987A are presented. Data are from the first two balloon flights of the Gamma-Ray Imaging Spectrometer. On day 433 (after the initial optical sighting) the 847 and 1238 keV lines from Co-56 were observed at 2.3 and 4.3 sigma significance. On day 613, the lines at 847, 1238, and 2599 keV were observed at 4.6, 3.4, and 1.9 sigma, respectively. The combined significance for the three-line complex in both flights is 7.8 sigma. Gaussian profiles yield acceptable least-squares fits to the lines. The line profiles are centered on the red side of the rest energy with typical velocity dispersions of about 3500 km/s FWHM, consistent with an optically thin source, but the line intensities are less than about 30 percent of those produced by the 0.075 solar mass of Co-56 determined from the bolometric light curve.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 351; L41-L44
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  • 7
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    Evidence for variability of the hard X-ray feature in the Hercules X-1 energy spectrum (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The hard X-ray spectrum of HER X-1 was measured for the first time with a high resolution (1.4 keV FWHM) germanium spectrometer. The observation was performed near the peak of the on-state in the 35 day cycle and the 1.24 pulsations were observed between the energies of 20 keV and 70 keV. The feature corresponds to an excess of 7.5 sigma over the low energy continuum. Smooth continuum models are poor fits to the entire energy range (chance probabilities of 2 percent or less). The best fit energies are 35 keV for an absorption line and 39 keV for an emission line. These are significantly lower energies than those derived from previous experiments. A direct comparison of the data with the results of the MPI/AIT group shows statistically significant variations which strongly suggest variability in the source. Previously announced in STAR as N83-37036
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 279; 177-183
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  • 8
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    GRIS observations of the Galactic Center (1990)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The 511 keV positron annihilation line source in the Galactic Center (GC) region has reappeared after being in a quiescent state since the early 1980's. Observations are reported made by the GRIS balloon instrument showing that the 511 keV line has returned to an intensity level similar to that seen in the 1970's. The line width was resolved for the first time and a measurement of the spatial extent of the emission along the galactic plane was made. Gamma ray imaging spectrometer (GRIS) is a high resolution germanium (Ge) spectrometer with a 17 deg field-of-view. Eleven hours of data were obtained from GC pointings on balloon flights over Australia on 1 May 1988 and again on 29 October 1988. An additional seven hours were obtained on 30 October 1988 from a point in the galactic plane (GP) 25 deg west of the center. Preliminary results for the line fluxes from the GC are 9.8 + or - 1.9 in May and 12.3 + or - 1.6 in October, and from the GP are 2.4 + or - 1.6 (1 sigma statistical errors). The flux for the off-center pointing is significantly lower than that for the GC pointings and indicates that the dominant emission is narrowly concentrated at the center. The line width for the GC pointing in October is 3.6 + or - 0.5 keV, which implies a temperature for the annihilation medium of less than or + 10(exp 5)K. A step in the continuum emission at 511 keV is found in both the GC and GP data. The step may be due to orthopositronium three-photon annihilation for the GC, but is too large relative to the 511 keV line for the GP to be simply explained by positronium.
    Keywords: ASTROPHYSICS
    Type: Annihilation in Gases and Galaxies; p 159-168
    Format: application/pdf
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  • 9
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    Gamma-ray burster counterparts - Infrared (1987)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The discovery of an IR counterpart to a gamma-ray burster (GRB) would be a big step forward in understanding this enigmatic phenomenon. A program of searching for IR emission from the smallest GRB source regions has been initiated. Seven boxes have been searched with ground-based telescopes at a wavelength of 2.2 microns and 23 with the IRAS data base at wavelengths of 12, 25, 60, and 100 microns. No convincing candidates were identified. The most constraining result are for two error boxes where the K-band magnitude are found to be fainter than 19.03. This observation is compared with the flux predicted from either a companion star or an accretion disk. The new IR observations pose serious difficulties for several GRB models.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 313; 226-230
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  • 10
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    Gamma-ray spectroscopy of the galactic center region - Confirmation of the time variability of the positron annihilation line (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: (Previously announced in STAR as N82-27199)
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 260
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