Veritas Observations of Day-Scale Flaring of M 87 in 2010 April

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 0.16) 10 -11 cm -2 s -1 at energies above 350 GeV. In 2010 April, VERITAS detected a flare from M 87 with peak flux of (2.71 0.68) 10 -11 cm -2 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 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.