An isotropic, apparently diffuse flux of
γ rays was
discovered
with SAS-2 in the 40
- 200 MeV range
[45].
EGRET, improving and extending the SAS-2 result, measured isotropic
γ-ray
emission in the
30
MeV - 100 GeV range
[38]
with
F
intensity at 1 GeV at the level
of
1
keV/(cm2-s-sr), and with
F
spectral index
-0.10 ± 0.03
(Fig. 1a). The diffuse isotropic
γ-ray
background consists of an extragalactic
γ-ray
background and an uncertain contribution of quasi-isotropic Galactic
γ rays
produced, for example, by Compton-scattered radiations from cosmic-ray
electrons. The model-dependent Galactic contribution
[38,
42,
43,
44],
and the addition at some level of heliospheric flux
[28,
34],
means that the actual contribution from extragalactic sources is
somewhat uncertain. The data in Fig. 1a compares
the extragalactic diffuse
γ-ray
intensity from EGRET analysis
[38]
with results using the GALPROP model
[44],
the latter of which requires an extended
(
4 - 10 kpc)
nonthermal electron halo to fit the hard
(
-0.4) diffuse Galactic
γ-ray
emission. For
our purposes, we consider the apparently diffuse extragalactic
γ-ray
background (EGRB) of
[44]
as the conservative upper limit for the superposed intensity of any class of
γ-ray
sources, with the
[38]
intensity as an absolute upper limit
to the combined residual intensity from all source classes.
![]() |
Figure 1. (a) Diffuse extragalactic γ-ray background from analyses of EGRET data, shown by filled [38] and open [42] data points, compared to model calculations of the contributions to the EGRB for FSRQs and BL Lac objects, and total AGNs [14], star-forming galaxies [35], starburst galaxies [46], structure shocks in clusters of galaxies [21, 6], and GRBs [12]. (b) Fitted EGRET and predicted redshift distributions of FSRQs and BL Lac objects [12]. (c) Fitted EGRET size distribution, and predictions for different flux levels [12]. |
The GALPROP fits
[42]
to the OSSE-COMPTEL-EGRET Milky Way intensity spectra in different
directions toward the Galaxy implies the total
γ-ray
luminosity of the Milky Way galaxy. Scaled to
1039 L39 ergs s-1, the GALPROP
analysis gives L39 = (0.71 - 0.92) for the > 100 MeV
γ-ray
luminosity of the Milky Way, a factor
3 greater than the
value L39 = (0.16 - 0.32) inferred from COS-B observations
[7].
Most of this emission is from secondaries created in cosmic-ray nuclear
production processes. The Galactic
γ-ray power
provides an important
yardstick to assess the total contribution of to the unresolved
γ-ray
background of cosmic-ray emissions from star-forming
galaxies, as described in more detail below.
Every γ-ray source class makes a different contribution to the γ-ray background, including transient events below detector threshold, variously oriented relativistic jet sources, and large numbers of individually weak sources. The basic formalism for making such calculations for beamed and unbeamed sources was given in my Barcelona talk [14]. Here I review the various source classes that likely dominate the composition of the diffuse background: blazars and radio / γ galaxies; star-forming galaxies of various types; γ rays from structure-formation shocks; and GRBs.