Session 28 - Diffuse Gamma-Ray Emission & Gamma-Ray Bursts.
Oral session, Monday, January 13
Harbour B,

[28.02] The Diffuse Cosmic Gamma-ray Background with SMM between 0.3 MeV and 8 MeV

K. Watanabe (NASA/GSFC/HSTX), M. D. Leising (Clemson U.), G. H. Share, R. L. Kinzer (NRL)

The cosmic diffuse \gamma-ray background (CGB) in the MeV region is extremely hard to measure. Some previous scintillation counter experiments have suggested a bump'' in the few Mev region, which was theoretically intriguing and hard to reconcile with proposed sources of the CGB in this energy region. The Solar Maximum Mission \gamma-ray spectrometer (SMM/GRS) had a large field of view (140^0 FWHM) and was sensitive from 0.3 Mev to 8.5 Mev. The largest contributor to the count rate in the SMM \gamma-ray detectors, after internal and Earth albedo backgrounds, was the isotropic extragalactic \gamma-ray emission. To extract this spectrum, the observed count rates were fitted with a model that is the sum of: 1) the variation of the CGB count rate, 2) the variation of the Earth albedo count rate, 3) the decay of radioactivity within the instrument and spacecraft and 4) cosmic rays (prompt and delayed). The CGB term of the model is the 4\pi integral of the GRS response minus the \sim4 steradian portion blocked by the Earth. Each energy channel was fitted independently, and in the end combined to form the entire spectrum. We use a non-linear fitting program to find non-linear parameters in the model (e.g. radioactive lifetimes). Brute force \chi^2 mapping was performed to locate the true global minimum in the multidimensional parameter space. We use a constrained linear fitting routine to explore the region near the global \chi^2 minimum in detail. The result shows no indication of the MeV bump. Our measured spectrum is roughly consistent with a power-law of photon index -2.90\pm0.1 energies 0.3 MeV to 3 MeV with a flux of 1.32(\pm 0.37) (photon cm^-2 s^-1 ster^-1 keV^-1) at 0.3 Mev. Near 1--3 MeV, this is much lower than past measurements and somewhat lower than the recent CGRO/COMPTEL measurement. We have only upper limits above 3 MeV.