Before the launch of Compton, only one extragalactic source, 3C 273, had been detected above 10 MeV (Bignami et al. 1979). So theorists tended to explain the origin of the high-energy GRB spectrum by diffuse mechanisms rather than by a superposition of unresolved discrete extragalactic sources. Above 100 MeV, this now has been changed by Compton observations of blazars (see Section 6), but in the 1-100 MeV range diffuse models are still the best interpretation. Below we describe two theoretical models that postulate a cosmological origin for the 1-100 MeV GRB.
5.1 GRB from Proton-Antiproton Annihilation
Stecker et al. (1971) proposed a diffuse emission model in which the GRB arises in a baryon symmetric Big Bang cosmology from matter-antimatter annihilations. In a grand-unified-theory model with spontaneous CP violation (Stecker 1985), it is possible for the Universe to have evolved very large regions of pure matter and pure antimatter containing masses the size of galaxy clusters or superclusters in its early history. These regions are essentially the fossils of the vanished CP domains. In a baryon symmetric cosmology, annihilations occur at the boundaries between these regions at all redshifts to produce an extragalactic GRB. Puget (1973) has computed the annihilation rate as a function of redshift. The annihilations produce °-mesons with gamma-ray producing decay modes. (° decay is also the principal mechanism for producing the galactic high-energy diffuse gamma radiation.) Figure 6 shows a typical rest-frame spectrum produced by proton-antiprotron annihilation with ° decay (Stecker 1971), with maximum intensity at m c2 / 2 ~ 70 MeV. The spectrum is nearly flat near the maximum, with a minimum energy of ~ 5 MeV and a maximum cutoff at ~ 1 GeV. To arrive at the predicted GRB spectrum observed at the current epoch, it is necessary to solve a cosmological photon transport equation and take into account pair production and Compton scattering at high redshifts which may cause energy loss (Stecker et al. 1971). The resultant spectru (Stecker 1989) matches the observed steep slope of the extragalactic GRB above ~ 1 MeV very well. Furthermore, it can account for the observed MeV bump (Fig. 7). Absorption due to Compton scattering and pair production causes the spectrum to bend over below ~ 1 MeV.
Figure 6. Gamma-ray spectrum from proton-antiproton annihilation at rest (Stecker 1971).
5.2 GRB from Primordial Blackholes
Page & Hawking (1976) postulated a population of primordial black holes (PBH) created in the early Universe that could explain the GRB. The PBHs cannot be created in the present epoch since the necessary compressional forces do not exist. They undergo quantum mechanical decay by radiating gravitons, neutrinos, electrons, positrons, and gamma rays. PBHs with initial masses less than a critical mass of 5 x 104 g would have completely evaporated by now. PBHs of slightly greater initial mass would be radiating energy at the rate of 2.5 x 1017 ergs/s. As they reach the end of their life, they evaporate by ejecting all remaining rest mass in a very short time. The particles emitted in this final release would decay rapidly (t << 1 sec), giving a short burst of gamma-rays between 100 MeV and 1 GeV. The EGRET instrument onboard Compton may be able to detect PBH bursts with this particular timing signature, but has not seen any such events to date. A uniform distribution of PBHs of initial mass 5 x 104 g, when integrated over the cosmological time scale, would give a GRB photon spectrum of power law index ~ 3 above 120 MeV. This matches the observed GRB spectral slope in the 100 MeV range rather well. Below 120 MeV, the spectrum may flatten depending on the space density of PBHs.
Figure 7. Contribution to the extragalactic background by proton-antiproton annihilations in a baryon-symmetric universe (Stecker et al. 1971).