The nature of the Diffuse Gamma-Ray Background

Abstract

We review the current understanding of the diffuse gamma-ray background (DGRB). The DGRB is what remains of the total measured gamma-ray emission after the subtraction of the resolved sources and of the diffuse Galactic foregrounds. It is interpreted as the cumulative emission of sources that are not bright enough to be detected individually. Yet, its exact composition remains unveiled. Well-established astrophysical source populations (e.g. blazars, misaligned AGNs, star-forming galaxies and millisecond pulsars) all represent guaranteed contributors to the DGRB. More exotic scenarios, such as dark matter annihilation or decay, may contribute as well. In this review, we describe how these components have been modeled in the literature and how the DGRB can be used to provide valuable information on each of them. We summarize the observational information currently available on the DGRB, paying particular attention to the most recent measurement of its intensity energy spectrum by the Fermi LAT Collaboration. We also discuss the novel analyses of the auto-correlation angular power spectrum of the DGRB and of its cross-correlation with tracers of the large-scale structure of the Universe. New data sets already (or soon) available are expected to provide further insight on the nature of this emission. By summarizing where we stand on the current knowledge of the DGRB, this review is intended both as a useful reference for those interested in the topic and as a means to trigger new ideas for further research

Tomographic Imaging of the Fermi-LAT γ-Ray Sky through Cross-correlations: A Wider and Deeper Look

“…2. The fact that in various cases a significant variation of the signal as a function of energy and redshift is observed strongly suggests that the UGRB is produced by different types of sources, as indicated also by recent population studies of resolved γ-ray sources Fornasa & Sanchez-Conde 2015 ] GeV. These dissimilarities reflect the differences in the relative bias between γ-ray sources and galaxies in the various catalogs, that is, the fact that different types of galaxies are more or less effective tracers of the unresolved γ-ray sources.…”

Modelling the flux distribution function of the extragalactic gamma-ray background from dark matter annihilation

“…Independently of this, dark matter has emerged as the preferred explanation of many astrophysical and cosmological features through gravity (galactic rotation curves, Ω m Ω b , lensing by galaxy clusters, etc.). If particle dark matter produces gamma rays (e.g., by self-annihilation) as in the case of weakly interacting massive particles (WIMPs) motivated by popular particle-physics models [10,11], then it could also contribute to this diffuse signal (in some unknown proportion) [12]. Given that the known astrophysical sources yield reasonable fit to the spectrum of the gamma-ray background, the dark matter component started to be tightly constrained only through the spectral data (e.g., [5,8,13]).…”

Statistical measurement of the gamma-ray source-count distribution as a function of energy

“…[7]), well-matching our findings. It can be seen that the 1pPDF method extends the source-population sensitivity by almost one order of magnitude below the nominal catalog detection thresholds 1 . Upper limits on a second (first, for the highest energy band) intrinsic break of dN/dS are depicted by the blue arrows in Fig.…”

“…[1] for a recent review) comprises pivotal properties of the high-energy gamma-ray sky. Its composition is investigated by discriminating individual sources from diffuse components, utilizing gamma-ray measurements from spaceborne or ground-based instruments.…”