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). |