As yet no experiment, satellite-borne, balloon-borne or earth-based has provided compelling evidence for more than upper limits to the intensity of cosmic gamma rays of more than a few MeV energy. Even these upper limits have been useful in blocking in some of the large scale properties of energetic particles in interstellar and intergalactic space. Nevertheless, the smallest upper limit set on the intensity of diffuse gamma rays 3.10−4 cm−2 s−1 sterad−1 (from the satellite experiment in Explorer XI) is a factor of about 20 above the intensity prediction which can be made with rather good confidence for gamma rays made in cosmic ray collisions with interstellar atomic hydrogen.
Predictions of the gamma ray flux from the various discrete-source emitters of synchrotron radio noise are model-sensitive and in general appreciably smaller than existing upper limits. These upper limits are in the 3.10−4 cm−2 s−1 region for gamma rays of E > 5.107 eV and in the 5.10−11 cm−2 s−1 region for gamma rays of E > 5.1012 eV.