Analysis Methods for Gamma-Ray Astronomy

The launch of the Fermi satellite in 2008, with its Large-Area Telescope (LAT) on board, has opened a new era for the study of gamma-ray sources at GeV (109 eV) energies. Similarly, the commissioning of the third generation of imaging atmospheric Cherenkov telescopes (IACTs) – H.E.S.S., MAGIC, and VERITAS – in the mid-2000s has firmly established the field of TeV (1012 eV) gamma-ray astronomy. Together, these instruments have revolutionized our understanding of the high-energy gamma-ray sky, and they continue to provide access to it over more than six decades in energy. In recent years, the ground-level particle detector arrays HAWC, Tibet, and LHAASO have opened a new window to gamma rays of the highest energies, beyond 100 TeV. Soon, the next-generation facilities such as CTA and SWGO will provide even better sensitivity, thus promising a bright future for the field. In this chapter, we provide a brief overview of methods commonly employed for the analysis of gamma-ray data, focusing on those used for Fermi-LAT and IACT observations. We describe the standard data formats, explain event reconstruction and selection algorithms, and cover in detail high-level analysis approaches for imaging and extraction of spectra, including aperture photometry as well as advanced likelihood techniques.