Published in Physics Reports, Volume 598, p. 1-58, 2015.
https://arxiv.org/abs/1502.02866

For a PDF version of the article, click here.

THE NATURE OF THE DIFFUSE GAMMA-RAY BACKGROUND

Mattia Fornasa


School of Physics and Astronomy, University of Nottingham, University Park NG7 2RD, UK

Miguel A. Sánchez-Conde


Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden


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.


Table of Contents

INTRODUCTION

THE INTENSITY ENERGY SPECTRUM
The new Fermi LAT measurement of the Diffuse Gamma-Ray Background
The astrophysical components of the Diffuse Gamma-Ray Background
Blazars
Misaligned Active Galactic Nuclei
Star-forming galaxies
Millisecond pulsars
Other astrophysical components
The Dark Matter component of the Diffuse Gamma-Ray Background
The case of annihilating Dark Matter
The case of decaying Dark Matter

THE ANGULAR POWER SPECTRUM OF ANISOTROPIES
The Fermi LAT measurement of gamma-ray anisotropies
Deducing the nature of the DGRB from its anisotropies

THE PHOTON COUNT DISTRIBUTION

THE CROSS-CORRELATION WITH INDEPENDENT PROBES
The cross-correlation with galaxy catalogs
The cross-correlation with cosmic shear
The cross-correlation with other tracers

THE SCIENCE OF THE DIFFUSE GAMMA-RAY BACKGROUND

REFERENCES

Table of acronyms

1FGL Fermi LAT First Source Catalog
2FGL Fermi LAT Second Source Catalog
2FPC Fermi LAT Second Catalog of gamma-ray pulsars
2LAC Fermi LAT Second Catalog of Active Galactic Nuclei
2MASS 2 Micron All-Sky Survey
3FGL Fermi LAT Third Source Catalog
AGN Active Galactic Nucleus
APS Angular Power Spectrum
CFHTLenS Canada-France-Hawaii Telescope Lensing Survey
CMB Cosmic Microwave Background
CR Cosmic Ray
CTA Cherenkov Telescope Array
DES Dark Energy Survey
DESI Dark Energy Spectroscopic Instrument
DGRB Diffuse Gamma-Ray Background
DM Dark Matter
eBOSS extended Baryon Oscillation Spectroscopy Survey
EBL Extragalactic Background Light
FRI Fanaroff-Riley Class I
FRII Fanaroff-Riley Class II
FSRQ Flat-Spectrum Radio Quasar
HMF Halo Mass Function
HOD Halo Occupation Distribution
HSP High-Synchrotron Peak
IC Inverse Compton
IR InfraRed
ISP Intermediate-Synchrotron Peak
JWST James Webb Space Telescope
LF Luminosity Function
LOFAR Low-Frequency Array
LSP Low-Synchrotron Peak
LSS Large-Scale Structure
MAGN Misaligned Active Galactic Nucleus
MSP MilliSecond Pulsar
MW Milky Way
NFW Navarro-Frenk-White
NuSTAR Nuclear Spectroscopic Telescope Array
NVSS NRAO VLA Sky Survey
PDF Probability Distribution Function
PSF Point Spread Function
SDSS Sloan Digital Sky Survey
SED Spectral Energy Distribution
SFG Star-Forming Galaxy
SFR Star-Forming Rate
SKA Square Kilometer Array
UHECR Ultra-High-Energy Cosmic Ray
UV UltraViolet
WIMP Weakly Interacting Massive Particle
WISE Wide-field Infrared Survey Explorer
WMAP Wilkinson Microwave Anisotropy Probe

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