The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV γ-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.

LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy -rays. The system was observed over several orbital cycles (between 2006 September and 2007 February) with the VERITAS array of imaging air Cerenkov telescopes. A signal of -rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of À ¼ 2:40 AE 0:16 stat AE 0:2 sys and a flux above 300 GeV corresponding to 15%-20% of the flux from the Crab Nebula.

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 -rays. Here we report the detection of -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 -ray emission is measured to be pointlike with an intrinsic source radius less than 4.5 0 . The differential energy spectrum is fitted well by a power-law function: dÈ/dE ¼ (7:4 AE 1:3 stat AE 1:5 sys )(E/TeV) (À2:31AE0:17 stat AE0:2 sys ) 10 À9 m À2 s À1 TeV À1 . We show strong evidence for a year-scale correlation between the -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 -ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.

We present evidence that the very-high-energy (VHE, E > 100 GeV) gamma-ray emission coincident with the supernova remnant IC 443 is extended. IC 443 contains one of the best-studied sites of supernova remnant/molecular cloud interaction and the pulsar wind nebula CXOU J061705.3+222127, both of which are important targets for VHE observations. VERITAS observed IC 443 for 37.9 hours during 2007 and detected emission above 300 GeV with an excess of 247 events, resulting in a significance of 8.3 standard deviations (σ) before trials and 7.5 σ after trials in a point-source search. The emission is centered at 6 h 16 m 51 s + 22°30 ′ 11 ′′ (J2000) ±0.03°s tat ± 0.08°s ys , with an intrinsic extension of 0.16°± 0.03°s tat ± 0.04°s ys . The VHE spectrum is well fit by a power law (dN/dE = N 0 × (E/TeV) −Γ ) with a photon index of 2.99 ± 0.38 stat ± 0.3 sys and an integral flux above 300 GeV of (4.63 ± 0.90 stat ± 0.93 sys ) × 10 −12 cm −2 s −1 . These results are discussed in the context of existing models for gamma-ray production in IC 443. Subject headings: gamma rays: observations -ISM: individual (IC 443 = VER J0616.9+2230 = MAGIC J0616+225)