Diffuse gamma-ray emission: Galactic and extra- galactic

Here is reviewed our current understanding of Galactic and extragalactic diffuse γ-ray emission. The spectrum of the extragalactic γ-ray background above 30 MeV can be well described by a power law with photon index α=2.1. In the 2-10 MeV range a preliminary analysis of COMPTEL data indicates a lower intensity than previously found, with no evidence for an MeV bump. Most of the models of a truly diffuse background seem to be in conflict with the observed spectrum. Though AGN are the most likely input from discrete sources, two independent attempts to model the high energy background as the superposition of unresolved AGN indicate that AGN underproduce the observed intensity. Therefore the origin of the extragalactic γ-ray background is still unknown. The Galactic diffuse γ-ray continuum is more intense than expected both at very low energies (≤ 100 keV) and at high energies (≥ 1 GeV). The published models for these excesses all involve cosmic ray electron interactions. While the low energy excess may have something to do with in-situ acceleration of electrons, the excess at high energies may be understood if the sources of cosmic ray electrons are discrete. The measured energy spectrum of the diffuse Galactic γ-ray continuum radiation thus may provide new insights into the acceleration of cosmic rays. The extragalactic gamma-ray background The extragalactic diffuse emission at γ-ray energies has interesting cosmological implications since the bulk of these photons suffer little or no attenuation during their propagation from the site of origin. Before the launch of the Compton Gamma-Ray Observatory (CGRO), several balloon experiments (White et al. 1977; Schönfelder et al. 1980) and the γ-ray spectrometer flown aboard three Apollo flights (Trombka et al. 1977) showed the presence of a feature in the few MeV range, that was in excess of the extrapolated hard X-ray continuum. At higher energies, above 35 MeV, the SAS-2 satellite provided the first clear evidence for the existence of an extragalactic γ-ray component (Fichtel et al. 1975). The first all-sky survey in low energy γ-rays (1 MeV – 30 MeV) has been performed by COMPTEL and at higher energies, above 30 MeV, by EGRET on board CGRO. The improved sensitivity, low instrumental background and a large field of view of these instruments have resulted in significantly improved measurements of the extragalactic γ-ray background. In the following I shall briefly summarize the recent 16th ECRS GR2 Lecture Page 2 analysis results, and then I shall discuss the implications of these new findings on the origin of the extragalactic diffuse emission and current models thereof. COMPTEL results (1-30 MeV) In the 1-10 MeV band diffuse studies tend to be complicated by difficulties in fully accounting for the instrumental background, which is in general composed of ‘prompt’ and ‘long-lived’ components. The prompt background is instantaneously produced by interactions of energetic particles in the spacecraft and thus it modulates with the instantaneous local cosmic-ray flux which is monitored by the veto dome. A linear extrapolation to zero veto count rate is used to eliminate the prompt background contribution. The long-lived background is caused by de-excitations of activated radioactive isotopes with long half lives, for which the decay rate is not directly related to the instantaneous cosmic ray flux. The long-lived background events are identified by their characteristic decay lines in the detector spectra. Monte-Carlo simulations of the isotope decay are then used to determine the absolute contribution of each of the isotopes. 10-1 100 101 102 103 104 105 106 107 108 109 Photon Energy (keV) 0.1 1.0 10.0 100.0 E 2 d J/ dE ( ke V 2 /( cm 2 -s -k eV -s r) HEAO A2,A4(LED)