3. THE EGRB BETWEEN 0.5 AND 30 MEV

The explanation for the origin of the EGRB at energies in the range of several MeV must be a non-blazar explanation. The reason for this is that while the EGRB spectrum in this energy range appears to be softer than that at higher energies [2], the data from OSSE and COMPTEL on individual blazars in this energy range indicate a harder spectrum than that at higher energies. The measured blazar spectra appear to break below ~ 10 MeV to spectra with a typical power-law index of ~ 1.7 [20]. Thus, even if unresolved blazars account for almost all of the EGRB in the 0.1 to 10 GeV range, this cannot be the case at lower energies.

Calculations have shown that a superposition of redshifted lines from Type Ia and Type II supernovae should reasonably provide a significant component of the EGRB at energies ~ 1 MeV. The important line emission is from the decay chain ⁵⁶Ni -> ⁵⁶Co -> ⁵⁶Fe and also from the decay of ²⁶Al, ⁴⁴Ti and ⁶⁰Co [21], [22]. However, supernovae cannot account for the entire EGRB in this energy range, since they produce no line emission above 3.5 MeV.

Another serious possibility as a significant contributer to the multi-MeV EGRB is non-thermal tails in the energy spectra of the AGN [23]. These would be the same AGN which have recently been resolved out by the Chandra telescope and found to be the dominant component of the once unresolved X-ray background [24].

A recent discussion of AGN models fitting the X-ray background has been given in Ref. [25]. While there are no data on individual AGN in the multi-MeV energy range at the present time, Stecker, Salamon and Done [23] have pointed to the galactic black hole candidate Cyg. X-1 as an example of a black hole source which has been shown from COMPTEL data to have a non-thermal tail extending to multi-MeV energies [26]. If the extragalactic black hole sources which make up the X-ray background have such non-thermal tails, they may account for most of the EGRB in the multi-MeV range.

It should be noted that the extraction of the ~ MeV EGRB from the raw COMPTEL data is a difficult process, in part owing to the fact that this double Compton scattering telescope was not designed to measure this background. In our opinion, a dedicated low-mass, free flyer satellite, specifically designed to measure the EGRB at low -ray energies will be required in order to accurately determine its characteristics.