Gamma-Ray Astronomy: From Fermi Up To The Hawc High-Energy Gamma-Ray Observatory In Sierra Negra

Gamma-rays represent the most energetic electromagnetic window for the study of the Universe. They are studied both from space at MeV and GeV energies, with instruments like the Fermi gamma-ray Space Telescope, and at TeV energies with ground based instruments profiting of particle cascades in the atmosphere and of the Cerenkov radiation of charged particles in the air or in water. The Milagro gamma-ray observatory represented the first instrument to successfully implement the water Cerenkov technique for gamma-ray astronomy, opening the ground for the more sensitive HAWC gamma-ray observatory, currently under development in the Sierra Negra site and already providing early science results.