1.1. COMPTEL Results (1-30 MeV)
In the 1-10 MeV band diffuse studies tend to be complicated by difficulties in fully accounting for the instrumental background, which is in general composed of ``prompt'' and ``long-lived'' components.
The prompt background is instantaneously produced by interactions of energetic particles in the spacecraft and thus it modulates with the instantaneous local cosmic-ray flux which is monitored by the veto dome. A linear extrapolation to zero veto count rate is used to eliminate the prompt background contribution.
The long-lived background is caused by de-excitations of activated radioactive isotopes with long half lives, for which the decay rate is not directly related to the instantaneous cosmic ray flux. The long-lived background events are identified by their characteristic decay lines in the detector spectra. Monte-Carlo simulations of the isotope decay are then used to determine the absolute contribution of each of the isotopes.
Figure 1. Multiwavelength spectrum of the diffuse extragalactic emission from X-rays to -rays taken from Sreekumar et al. (1997). The estimated contributions from Seyfert I (dot-dashed) and Seyfert II (dashed) are from the model by Zdiarski (1996); steep-spectrum quasar contribution (triple dot-dashed) is taken from Chen et al. (1997); Type Ia supernovae (dotted) is from The et al. (1993). Also included is a possible blazar contribution (long dashed) assuming an average power law index of -1.7 below 4 MeV (McNaron-Brown et al. 1995) and -2.15 at higher energies (Mukherjee et al. 1997). The thick solid line indicates the sum of all components.
The diffuse flux measured by COMPTEL still includes contributions from the Galactic diffuse emission and -ray point sources in the field of view. The results below 9 MeV should, anyway, be considered preliminary. Nevertheless it is clear to date that the 2-9 MeV flux is significantly lower than measured in the pre-COMPTEL era. There is no evidence for an MeV-bump (Kappadath et al. 1996), a result supported also by a recent analysis of SMM data (Watanabe et al. 1997). The 9-30 MeV flux is compatible with earlier measurements and also with the extrapolation of the EGRET spectrum. The measured 9-30 MeV spectra from the Virgo and South Galactic Pole regions are consistent with each other and hence with an isotropic nature for the diffuse radiation (Kappadath et al. 1997). The new results are shown in Figure 1 compared with the earlier measurements.