A truly diffuse flux of
γ 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
γ
γ
processes, this intensity will be well below the
Waxman-Bahcall intensity at
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 γ
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
γ-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
(
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
γ-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
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
γ rays
are required to explain the
γ-ray
background.
Acknowledgments
The work of C. D. D. is supported by the Office of Naval Research and NASA GLAST Science Investigation No. DPR-S-1563-Y.