Discovery of fast variability of the TeV γ-ray flux from the giant radio galaxy M87 with H.E.S.S.

Abstract

The giant radio galaxy M 87 was observed at GeV/TeV γ‐ray energies with the H.E.S.S. (High Energy Stereoscopic System) Cherenkov telescopes in the years 2003–2006. The observations confirm M 87 as the first extragalactic TeV γ‐ray source not of the blazar type (first indications of a signal were reported by the HEGRA collaboration earlier). The TeV γ‐ray flux as measured with H.E.S.S. is variable on time‐scales of days which strongly constrains the size of the emission region, excluding several models. The H.E.S.S. results and theoretical interpretations are presented in this paper.