R. Lingenfelter

Compton-backscattered annihilation radiation from the Galactic Center region

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

Observation of SN 1987A with the gamma-ray spectrometer HEXAGONE

The HEXAGONE balloon-borne spectrometer was flown from Alice Springs (Australia) on 1989 May 22. HEXAGONE is a high-resolution gamma-ray spectrometer and consists of an array of twelve cooled germanium detectors (field of view 19° at 511 keV). One of the observed targets was the supernova 1987A and it was seen during 9.9 hr, 818 days after the initial optical outburst. No significant hard X-ray or gamma-ray emission is detected in the final spectrum of SN 1987A

An observation of annihilation radiation from the Galactic Center region

The Galactic Center region was observed on 22 May 1989 with a high resolution gamma-ray spectrometer flown from Alice Springs, Australia. The instrument contained an array of 12 cooled Ge detectors, each 5.5 cm dia x 5.5 cm long, with an energy resolution of 2.2 keV at 0.5 MeV, which were collimated to 18° FWHM. The observation was made at approximately 4 gm/cm2 atmosphere depth for 6 hours in a series of target and background pointings lasting 20 minutes each. Results from multiparameter Gaussian fits to the data are: 511 keV flux = (8.9 ± 2.7) x 10−4 ph/cm2-sec, line width of 1.1 −1.1+1.4 keV, and < 3.2 keV FWHM at 95% confidence. If the flux is interpreted as diffuse galactic emission, the 24° effective aperture for a uniform source gives a flux of (2.12 ± 0.64) x10−3 ph/cm2-s-rad, consistent with the SMM observations of diffuse galactic flux. The flux and width are both less than the October 1988 measurements with a 17° FWHM instrument which were obtained when the compact source at the galactic center was known to be “on”. Thus the variable compact source in the Galactic Center region may be responsible for the wider 511 keV line emission. The line width limit is consistent with annihilation in the warm (104 K) phase of the interstellar medium.

An observation of the Galactic center region with the HEXAGONE high resolution gamma‐ray spectrometer

The galactic center region was observed for 6 hours on 22 May 1989 from a high altitude balloon with the HEXAGONE high resolution gamma-ray spectrometer. The instrument had a 285 cm{sup 2} array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18{degree} FWHM field of view. 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of {sup 26}Al were observed to have fluxes of 8.9{times}10{sup {minus}4} and 1.9{times}10{sup {minus}4} ph/cm{sup 2}-s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of {minus}2.6. In the 120--250 keV band the spectrum contains a broad line-like feature with a flux of (2 to 6){times}10{sup {minus}3} ph/cm{sup 2}-s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of {similar to}511 keV photons from a compact source of electron-positron annihilation radiation.

An Observation of SN1987A with a New High Resolution Gamma-Ray Spectrometer

The discovery (Matz et al. [1]) of gamma-ray line emission at 847 and 1238 keV from radioactive 56Co in the recent supernova SN1987A proved that explosive nucleosynthesis occurred in this supernova. Gamma-ray light curves derived from these and subsequent observations (Cook et al. [2], Mahoney et al. [3], Sandie et al. [4], Rester et al. [5], Teegarden et al. [6]) have a broad plateau from August 1987 to October 1988, with an 847 keV flux of ~ 7×l0-4 ph/cm2-sec (Tueller et al. [7]). The early detection of gamma rays required the inclusion of mixing or clumping in the models, (e.g. Pinto and Woosley [8] and Chan and Lingenfelter [9]). The gamma-ray fluxes are predicted, e.g. Bussard et al. [10], to peak at about day 400 and then decrease, by a factor of ~ 6 at day 800, as the effect of increasing transparency becomes dominated by radioactive decay. Then they should depend primarily on the amount of 56Co produced and little on the degree on mixing since most of the 56Co should be exposed. Thus measurements of the 56Co gamma-ray line fluxes and profiles will continue to be important during the decline of SN1987A.