VERITAS Observations of day-scale flaring of M87 in 2010 April

TL;DR

VERITAS observed a day-scale gamma-ray flare from M87 in April 2010, providing detailed spectral and temporal data that shed light on particle acceleration and emission region size.

Contribution

This study presents the first detailed analysis of a rapid VHE gamma-ray flare from M87, including spectral evolution and flux variability over a six-night observation period.

Findings

Detected a flare with a peak flux of (2.71 +- 0.68) x 10^{-11} cm^{-2} s^{-1}

Measured flux decay time of approximately 0.9 days

Spectral index at peak is 2.19 +- 0.07, with indications of spectral softening during decay

Abstract

VERITAS has been monitoring the very-high-energy (VHE; >100GeV) gamma-ray activity of the radio galaxy M87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with a peak flux of (0.70 +- 0.16) X 10^{-11} cm^{-2} s^{-1} at energies above 350GeV. In 2010 April, VERITAS detected a flare from M87 with peak flux of (2.71 +- 0.68) X 10^{-11} cm^{-2} s^{-1} for E>350GeV. 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 M87 and the constraints they provide on the physical size of the emitting region.