Fast variability of Tera-Electron Volt gamma-rays from the radio galaxy M87

The detection of fast variations of the TeV (10^12 eV) gamma-ray flux, on time-scales of days, from the nearby radio galaxy M 87 is reported. These variations are ~10 times faster than that observed in any other waveband and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma-rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed towards the observer.