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A truly diffuse flux of gamma rays will be formed by the cascade radiations initiated by photopion and photopair production of ultra-high energy cosmic rays interacting with photons of the extragalactic background light. Because the electromagnetic secondaries are distributed over several orders or magnitude as they cascade to photon energies where the universe becomes transparent to gamma gamma processes, this intensity will be well below the Waxman-Bahcall intensity at approx 0.03 keV/(cm2-s-sr). By comparing with the diffuse neutrino intensities calculated in bottom-up scenarios for the ultra-high energy cosmic rays [13, 52], cosmogenic gamma rays are not expected to make a large contribution to the EGRB (however, see Ref. [20], though they could for top-down models [37]. This question will definitively be answered by Auger data.

The various source classes that contribute to the extragalactic gamma-ray background have hardly been exhausted, but the described classes are expected to be most important. Yet when one adds up the best guesses of the various contributions to the total, as shown in Fig. 1a, a deficit remains at both low (ltapprox 100 MeV) and high (≫ 1 GeV) energies. Because star-forming and starburst galaxies make such a large contribution to the total, it is possible that their spectra are actually much softer than assumed on the low-energy side, due to nonthermal electron bremsstrahlung and Compton-scattered emissions from gamma-ray production by cosmic rays in "thick-target" starburst and infrared luminous galaxies (cf. [32]). This, or soft-spectrum radio galaxies and from the superposition of hard tailes from many weak radio-quiet Seyfert galaxies, could explain the low-energy deficit.

It seems unlikely, however, that star-forming galaxies, whose high-energy radiation originates from cosmic rays accelerated by supernova remnant shocks, could explain the deficit on the high-energy side unless shock injection spectra harder than -2 were postulated. The EGRET effective area dropped rapidly above approx 5 GeV due to self-vetoing effects, so it was not sensitive to hard-spectrum sources, in particular, hard spectrum BL Lac objects. But the BL Lac contribution is estimated at the 5% level, and it is difficult to suppose that EGRET was not able to detect a number of such hard-spectrum BL Lac objects. Hard tails on FSRQs originating, e.g., from photohadronic cascade emissions [2], could explain the high-energy discrepancy. Other possibilities are the diffuse contributions from dark matter annihilation [49], or cascade radiations from misaligned blazars [1]. We must furthermore keep in mind the possibility that the model of foreground Galactic emission that must be subtracted from the extragalactic flux is incomplete [22], or that the EGRET internal background was underestimated [3]. Data from GLAST will tell us which, if any, of these suggestions are correct, and whether new, unexpected sources of high-energy gamma rays are required to explain the gamma-ray background.


The work of C. D. D. is supported by the Office of Naval Research and NASA GLAST Science Investigation No. DPR-S-1563-Y.

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