Observation of Gamma‐Ray Emission from the Galaxy M87 above 250 GeV with VERITAS1

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: d Φ/ 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.