The first measurement of the GRB was obtained by a CsI scintillation detector onboard the Ranger 3 Moon probe in 1962 in the energy range of 0.1 to 3 MeV (Arnold et al. 1962). The lunar missions Apollo 15, 16 & 17 flown in the early 1970's carried identical NaI scintillation spectrometers and obtained data in the 0.3 to 10 MeV energy range (Trombka et al. 1977). The measured spectrum was well-fitted by an electron bremsstrahlung model, after subtracting the unwanted background components caused by charged particle activation of the detector and the spacecraft. The results agreed with other data points (Table 1) within experimental uncertainties and connected smoothly to high-energy gamma-ray measurements by SAS-2 in 1972 (Fichtel et al. 1975) (Fig. 2). There was some indication of a flatter slope in the 1 to 5 MeV region (Fig. 1).
HEAO-1 was launched in 1977 with one of its primary objectives to measure the spectrum of the cosmic diffuse background. The A-2 and A-4 experiments together spanned the X-ray to low-energy gamma ray regime from 3 keV to 4.0 MeV. The two experiments used different techniques, yet obtained two spectra that joined smoothly and fitted remarkably well to a single thermal bremsstrahlung model of kT ~ 40 keV in the transitional energy range of 60 to 100 keV (Fig. 1) (Marshall et al. 1980; Gruber et al. 1985). Besides measuring the GRB, the A-4 experiment also detected about 50 X-ray sources in the Galactic plane (Levine et al. 1984), the diffuse Galactic ridge that extends approximately ±50° in longitude and ±5° in latitude (e.g., Gehrels & Tueller 1993 and references therein), and around 20 active galactic nuclei (AGNs). The AGNs have simple power-law spectra with energy indices clustered around the value of ~ 0.7 (Mushotzky 1982, 1984; Rothschild et al. 1983).
There was some evidence in the A-4 data of spectral flattening around a few MeV (Fig. 1). Data obtained by balloon-borne Compton telescopes (Schönfelder et al. 1980; White et al. 1977) in the energy range of 1 to 20 MeV were consistent with the existence of the MeV bump. Beyond 5 MeV, the GRB spectrum steepens to a power law of energy index ~ 2 (Schönfelder et al. 1980), in excellent agreement with the extrapolation of SAS-2 measurements (Fichtel et al. 1978) towards lower energies.
Gruber (1992) has performed a best fit after reviewing the available spectral data published prior to 1990 (Fig. 1). He found that below 60 keV, the diffuse background energy flux was fitted well by a bremsstrahlung spectrum. But above 60 keV, he needed to add to the bremsstrahlung extension a power law component with an index of ~ 0.7, which characterizes the energy spectra of AGN measured by HEAO-1. His empirical functional fit to the energy flux in units of keV per cm2-s-keV-sr is:
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Comprehensive studies have been performed on the spatial structure of the low-energy GRB using HEAO-1 data (Boldt 1987). The HEAO-1 detectors had nominal fields-of-view of 3° x 3°, 3° x 1.5°, and 3° x 6°. Shafer (1983), using the A-2 HED data from 2.5 to 60 keV, derived a dipole amplitude of dI/I0 = (0.5 ± 0.2)%, with a peculiar velocity v = 475 ± 165 km/sec and apex at l = 282°, b = 30°. Gruber (1992), using the A-4 MED data from 95 to 165 keV, found a similar dipole anisotropy with dI / I0 = (2.2 ± 0.7)%, v = 1450 ± 440 km/sec and apex at l = 304°, b = 26°. These results are consistent, within the experimental uncertainties, with each other and also with the recent microwave result from COBE (Smoot et al. 1992; Kogut et al. 1993), which gives a peculiar velocity v = 627 ± 22 km/s, and apex at l = 264°, b = 48°. Gruber noted that the large peculiar velocity measured in A-4 could be due to a local enhancement of the diffuse background.