The first all-sky survey of cosmic gamma rays of energies above 50 MeV
was carried
out by OSO-3 in 1967-1968. The directional scintillator/Cerenkov
detector recorded
621 "sky events" in 16 months of operations
(Fig. 4). The observations can be attributed
to three components: (1) a galactic component, concentrated along the
galactic plane
and well-correlated with the column density as deduced from 21-cm radio
measurements;
(2) a galactic center component; and (3) an isotropic, extragalactic component
with a steep power-law spectrum. Subsequent measurements made using an entirely
different technique - spark chamber detectors - confirmed the general
picture. These included the SAS-2
(Fichtel et al. 1975)
mission and several balloon flights
(Table 1).
SAS-2 measured a diffuse GRB component with a very steep
differential power law of
energy index 
~ 1.7 (photon
index = + 1 ~ 2.7) between 35
MeV and 200 MeV
(Fichtel et al. 1978)
(Fig. 5). The extrapolated intensity of this
component to lower
energies agreed well with measurements at 10 MeV. Above several hundred
MeV, the
flux of the extragalactic GRB falls below the galactic high latitude
background and the
determination of its value is highly dependent on data analysis techniques.
|
Figure 4. OSO-3 sky events plotted in galactic coordinates (Kraushaar, Clark & Garmire 1972). |
|
Figure 5. Differential photon spectrum of the SAS-2 diffuse gamma-ray background (Fichtel et al. 1978). |