Observation of Gamma-Ray Emission from the Galaxy M87 above 250 GeV with VERITAS

The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in active galactic nuclei from radio waves to TeV γ-rays. Here we report the detection of γ-ray emission above 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cerenkov telescope array and discuss its correlation with the X-ray emission. The γ-ray emission is measured to be pointlike with an intrinsic source radius less than 4.5^'. The differential energy spectrum is fitted well by a power-law function: dΦ/dE = (7.4 +/- 1.3_stat +/- 1.5_sys)(E/TeV)^{(- 2.31 +/- 0.17stat +/- 0.2sys)} 10^-9 m^-2 s^-1 TeV^-1. We show strong evidence for a year-scale correlation between the γ-ray flux reported by TeV experiments and the X-ray emission measured by the ASM RXTE observatory, and discuss the possible short-timescale variability. These results imply that the γ-ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.

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