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.

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5°. VERITAS will substantially increase the catalog of very high energy (E > 100 GeV) c-ray sources and greatly improve measurements of established sources. Ó

We present the discovery of >350 GeV gamma-ray emission from the BL Lacertae (BL Lac) object 1ES 2344+514 with the Whipple Observatory 10m gamma-ray telescope. This is the third BL Lac object detected at very high energies (VHE, E > 300 GeV), the other two being Markarian 421 (Mrk 421) and Mrk 501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified. The evidence for emission from 1ES 2344+514 comes mostly from an apparent flare on 1995 December 20 (universal date) during which a 6σ excess was detected with an average flux of I(>350 GeV) = 6.6 ± 1.9 × 10 −11 photons cm −2 s −1 . This is approximately 63% of the VHE emission from the Crab Nebula, the standard candle in this field. Observations taken between 1995 October

We report a possible detection of TeV gamma rays from the Galactic center by the Whipple 10 m gammaray telescope. Twenty-six hours of data were taken over an extended period from 1995 through 2003 resulting in a total significance of 3.7 j. The measured excess corresponds to an integral flux of Ϫ8 1.6 # 10 ‫ע‬ 0.5 # above an energy of 2.8 TeV, roughly 40% of the flux from the10 (stat) ‫ע‬ 0.3 # 10 (sys) photons m s Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15Ј and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence of variability.

1 Fred Lawrence Whipple Observatory, Harvard-Smithsonian CfA,

Exceptionally strong and long-lasting flaring activity of the blazar Mrk 421 occurred between 2001 January and March. Based on the excellent signal-to-noise ratio of the data, we derive the energy spectrum between 260 GeV and 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead, the data can be described by a power law with exponential cutoff: with TeV. Mrk 421 is the second g-ray blazar thatunambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV. Subject headings: BL Lacertae objects: individual (Markarian 421) -gamma rays: observationsSince the discovery of TeV g-rays from BL Lac objects Mrk 421 (Punch et al. 1992) and Mrk 501 (Quinn et al. 1996), detailed very high energy observations of these nearby blazars (z p , ) have been made. Measurements of flux var-0.031 z p 0.034 iation with time, particularly simultaneous measurements at several wavelengths, constrain models of particle acceleration and g-ray production in the jets. Spectral energy density measurements constrain both the models of the jets and of the infrared photon density in the intervening intergalactic medium. The possibility of absorption of g-rays by IR radiation has been predicted for some time (Nikishov 1962;Gould & Schrèder 1967;Stecker, De Jager, & Salamon 1992), and implications of recent observations have been discussed (see, e.g., Biller et al. 1998;Stanev & Franceschini 1998;Vassiliev 1999).The general picture that has emerged for the spectral energy density of emitted radiation from BL Lac objects has two components: a lower one with energies extended up to about

Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and Abell 2029 using the 10 m Whipple Cherenkov telescope during the 2003-2004 and 2004-2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeV emission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeV and theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of -2.1, our results imply that the cosmic ray proton energy density is less than 7.9% of the thermal energy density for the Perseus cluster.

We report on a multiwavelength campaign on the TeV -ray blazar Mrk 421 performed during 2002 December and 2003 January. These target of opportunity observations were initiated by the detection of X-ray and TeV -ray flares with the All Sky Monitor (ASM ) on board the Rossi X-Ray Timing Explorer (RXTE ) and the 10 m Whipple -ray telescope. The campaign included observational coverage in the radio (University of Michigan Radio Astronomy Observatory), optical (Boltwood, La Palma KVA 0.6 m; WIYN 0.9 m), X-ray (RXTE pointed telescopes), and TeV -ray (Whipple and HEGRA) bands. At TeV energies, the observations revealed several flares at intermediate flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula. While the time-averaged spectrum can be fitted with a single power law of photon index À ¼ 2:8 from dN /dE / E ÀÀ , we find some evidence for spectral variability. Confirming earlier results, the campaign reveals a rather loose correlation between the X-ray and TeV -ray fluxes. In one case, a very strong X-ray flare is not accompanied by a comparable TeV -ray flare. Although the source flux was variable in the optical and radio bands, the sparse sampling of the optical and radio light curves does not allow us to study the correlation properties in detail. We present a simple analysis of the data with a synchrotron self-Compton model, emphasizing that models with very high Doppler factors and low magnetic fields can describe the data.