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 a timescale as short as 1 day at a significance level of 5.6 \sigma .
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 \pm 0.11 _ { \mathrm { stat } } \pm 0.20 _ { \mathrm { syst } } .
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 .