VERITAS has been monitoring the very-high-energy ( VHE ; > 100 GeV ) gamma-ray activity of the radio galaxy M 87 since 2007 .
During 2008 , flaring activity on a timescale of a few days was observed with a peak flux of ( 0.70 \pm 0.16 ) \times 10 ^ { -11 } \mathrm { cm } ^ { -2 } \mathrm { s } ^ { -1 } at energies above 350 GeV .
In 2010 April , VERITAS detected a flare from M 87 with peak flux of ( 2.71 \pm 0.68 ) \times 10 ^ { -11 } \mathrm { cm } ^ { -2 } \mathrm { s } ^ { -1 } for E > 350 GeV .
The source was observed for six consecutive nights during the flare , resulting in a total of 21 hr of good quality data .
The most rapid flux variation occurred on the trailing edge of the flare with an exponential flux decay time of 0.90 ^ { +0.22 } _ { -0.15 } days .
The shortest detected exponential rise time is three times as long , at 2.87 ^ { +1.65 } _ { -0.99 } days .
The quality of the data sample is such that spectral analysis can be performed for three periods : rising flux , peak flux , and falling flux .
The spectra obtained are consistent with power-law forms .
The spectral index at the peak of the flare is equal to 2.19 \pm 0.07 .
There is some indication that the spectrum is softer in the falling phase of the flare than the peak phase , with a confidence level corresponding to 3.6 standard deviations .
We discuss the implications of these results for the acceleration and cooling rates of VHE electrons in M 87 and the constraints they provide on the physical size of the emitting region .