Abstract
M 87 is the only known non blazar radio galaxy to emit very high energy (VHE) gamma‐rays. During a monitoring program of M 87, a rapid flare in VHE gamma‐rays was detected by the MAGIC telescope in early 2008. The flux was found to be variable above 350 GeV on atimescaleas short as 1 day at a significance level of 5.6σ. The highest measured flux reached 15% of the Crab Nebula flux. We observed several substantial changes of the flux level during the 13 day observing period. The flux at lower energies (150–350 GeV), instead, is compatible with being constant. The energy spectrum can be described by a power law with a photon index of 2.30±0.11stat±0.20syst. The observed day‐scale flux variability at VHE prefers the M 87 core as source of the emission and implies that either the emission region is very compact (just a few Schwarzschild radii) or the Doppler factor of the emitting blob is rather large in the case of a non expanding emission region.
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