HEAD Division Meeting 1999, April 1999
Session 35. ISM and Supernova
Oral, Wednesday, April 14, 1999, 11:00am-12:31pm, Colonial Room

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[35.03] COMPTEL Measurements of the Cosmic Diffuse Gamma-Ray Spectrum from 800 keV to 30 MeV

S.C. Kappadath (U. of New Hampshire; presently: Louisiana State U.), M. McConnell, J. Ryan (U. of New Hampshire, USA), K. Bennett, C. Winkler (Astrophysics Division, ESTEC, The Netherlands), H. Bloemen, W. Hermsen (SRON-Utrecht, The Netherlands), V. Schoenfelder, M. Varendorff, G. Weidenspointner (MPE-Garching, Germany)

We report measurements of the Cosmic Diffuse Gamma Ray (CDG) spectrum from 800 keV to 30 MeV obtained with the Imaging Compton telescope COMPTEL, on-board the Compton Gamma Ray Observatory. The COMPTEL spectrum is consistent with power-law extrapolations of the CDG flux from lower and higher energies. The COMPTEL flux in the 1--10 MeV range is about 5--10 times lower than the pre-COMPTEL estimates and shows no evidence of an 'MeV-bump' in the 1--10 MeV range. The CDG spectrum was constructed by measuring the gamma-ray count rate from high galactic latitudes during periods when the Earth is outside the COMPTEL field-of-view. Special data selections were applied to suppress the prompt and delayed background contributions. The prompt background is produced primarily through interactions of neutrons and protons with the spacecraft, whereas the delayed emission comes from long-lived radioactivity. Above 4.2 MeV, where there is no long-lived background, the count rates were extrapolated to zero cosmic-ray intensity to estimate the prompt background and determine the count rate due to the CDG. The long-lived background was computed by fitting the energy spectrum. To determine the CDG count rates below 4.2 MeV, the long-lived background contributions were subtracted before extrapolating the count rates to zero cosmic-ray intensity.

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