VERITAS is a ground-based gamma-ray observatory that uses the imaging atmospheric Cherenkov technique and operates in the very high-energy (VHE) region of the gamma- ray spectrum from 100 GeV to 50 TeV. The observatory consists of an array of four 12m-diameter imaging atmospheric Cherenkov telescopes located in southern Arizona, USA. The four-telescope array has been fully operational since September 2007, and over the last two years, VERITAS has been operating with high reliability and sensitivity. It is currently one of the most sensitive VHE observatories. This paper summarizes the status of VERITAS as of October, 2009, and describes the detection of several new VHE gamma-ray sources.

VERITAS (Very Energetic Radiation Imaging Telescope Array System). is one of a new generation of TeV gamma-ray observatories. The current status of its construction is described here. The first two telescopes and cameras have been completed and meet the design specifications; the full array of four telescopes could be operational by the end of 2006.

Gamma-ray burst (GRB) observations at very high energies (VHE, E > 100 GeV) can impose tight constraints on some GRB emission models. Many GRB afterglow models predict a VHE component similar to that seen in blazars and plerions, in which the GRB spectral energy distribution has a double-peaked shape extending into the VHE regime. VHE emission coincident with delayed X-ray flare emission has also been predicted. GRB follow-up observations have had high priority in the observing program at the Whipple 10m Gamma-ray Telescope and GRBs will continue to be high priority targets as the next generation observatory, VERITAS, comes on-line. Upper limits on the VHE emission, at late times (> 4 hours), from seven GRBs observed with the Whipple Telescope are reported here.

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide range of flux. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales at both X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux did during a giant flare (or outburst) in 2004 (with the peak flux reaching ~135 mcrab in X-rays, as seen by the RXTE ASM, and ~3 crab in gamma rays). Such a difference in the development of the flare presents a further challenge to both the leptonic and hadronic emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in the optical seems to be comparable to that in the radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured spectral energy distributions (SEDs) with a one-zone synchrotron self-Compton model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

We report results from an intensive multi-wavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003–2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with Rossi X-ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broad-band spectral variability, over a wide range Department of Physics, Purdue University, West Lafayette, IN 47907, USA Department of Physics, University of Massachusetts, Amherst, MA 01003, USA Department of Physics, University of Leeds, Leeds, LS2 9JT, Yorkshire, England, UK Department of Physics, Washington University, St. Louis, MO 63130, USA Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Department of Physics, University of California, Los Angeles, CA 90095, USA Physics Department, National University of Ireland, Belfield, Dublin 4, Ireland Physics Department, Grinnell College, Grinnell, IA 50112, USA Adler Planetarium and Astronomy Museum, Astronomy Department, Chicago, IL 60605 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA Physics Department, National University of Ireland, Galway, Ireland High Energy Astrophysics Institute, University of Utah, Salt Lake City, UT 84112, USA Physics Department, McGill University, Montreal, QC H3A 2T8, Canada Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA Physics Department, DePauw University, Greencastle, IN, 46135, USA School of Physical Sciences, Galway-Mayo Institute of Technology, Galway, Ireland Department of Applied Physics and Instrumentation, Cork Institute of Technology, Cork, Ireland Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA Metsähovi Radio Observatory, Helsinki University of Technology, Mesähovintie 114, 02540 Kylmälä, Finland Boltwood Observatory, 1655 Main Street, Stittsville, Ont. K2S 1N6, Canada Department of Physics, University of Colorado at Denver, CO 80217, USA Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Department of Astronomy, Ohio University, Athens, OH 05701, USA Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany

We present results from observations of 29 BL Lacertae objects, taken with the Whipple Observatory 10 m gamma-ray telescope between 1995 and 2000. The observed objects are mostly at low redshift (z < 0.2), but observations of objects of up to z = 0.444 are also reported. Five of the objects are EGRET sources and two are unconfirmed TeV sources. Three of the confirmed sources of extragalactic TeV gamma rays were originally observed as part of this survey and have been reported elsewhere. No significant excesses are detected from any of the other objects observed, on timescales of days, months, or years. We report 99.9% confidence level flux upper limits for the objects for each observing season. The flux upper limits are typically 20% of the Crab flux, although for some sources, limits as sensitive as 6% of the Crab flux were derived. The results are consistent with the synchrotron self-Compton model predictions considered in this work.

We give an update on the status of the prototype telescope of the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a next-generation atmospheric imaging Cerenkov array currently under construction in southern Arizona. VERITAS will feature significant improvements over previous instruments, including better energy resolution, lower energy threshold, improved anpp. 2803–2806 c ©2003 by Universal Academy Press, Inc.

A survey of binary systems containing pulsars was conducted, with the intention of detecting Galactic sources of very high energy γ-ray emission. Observations were carried out with the Whipple 10 m imaging atmospheric Cerenkov telescope. Standard analysis techniques were applied to these sources to search for steady, unpulsed emission. Periodic tests were also performed to search for emission correlated with both the orbital and spin phases, where appropriate. Analyses indicate that the binaries in this study do not emit detectable levels of very high energy photons within the sensitivity of our instrument. The flux upper limits presented here fail to seriously constrain emission models.

The Whipple 10-m telescope was used to observe M87 since 2000. No significant gamma-ray signal was found and upper limits compared to the HEGRA detection suggest the source may be variable. We found weak evidence for a correlation with the X-ray activity in 2000-2001 but this tendency did not persist in 2002-2003.

At the Haverah Park Array a number of air shower observables were measured that are relevant to the determination of the mass composition of cosmic rays. In this paper we discuss measurements of the risetime of signals in large area water-Cherenkov detectors and of the lateral distribution function of the water-Cherenkov signal. The former are used to demonstrate that the CORSIKA code, using the QGSJET98 model, gives an adequate description of the data with a low sensitivity, in this energy range, to assumptions about primary mass. By contrast the lateral distribution is sufficiently well measured that there is mass sensitivity. We argue that in the range 0.2–1.0 EeV the data are well represented with a bi-modal composition of (34 ± 2)% protons and the rest iron. We also discuss the systematic errors induced by the choice of hadronic model.

At the Haverah Park Array a number of observables were measured that are relevant to the determination of the mass composition. In this paper we discuss measurements of the risetime of signals in large area water-Cherenkov detectors and of the lateral distribution function of the waterCherenkov signal. The former are used to demonstrate that the CORSIKA code, with QGSJET physics, gives an adequate description of the data with a low sensitivity, in this energy range, to assumptions about primary mass. By contrast the lateral distribution is sufficiently well measured that there is mass sensitivity and we argue that in the range 0.30.5 EeV the data are well represented with a bi-modal composition of 30% protons and 70% iron.

The energies of air showers recorded at the Hav- erah Park Array have been re-estimated using the CORSIKA program. As in the original analysis the cosmic ray energy was determined via its relation to the ground-level parame- ter (600). This relation was obtained previously through simulations with a rather simple hadronic interaction model. In this work we use CORSIKA with QGSJET to model air showers and GEANT to simulate the detailed detector re- sponse to ground particles, leading to a modified relation between (600) and the primary energy. A revised energy spectrum is reported for 3 10 17 eV< E < 4 10 18 eV.

Air showers recorded by the Haverah Park Array during the years 1974 - 1987 have been re-analysed. For the original estimate of the energy spectrum, a relationship between the ground parameter $ ho$(600) and the primary energy, as determined by Hillas in the 1970s, was used. Here we describe the energy spectrum obtained using the QGSJET98 interaction model in the CORSIKA Monte Carlo code, together with GEANT to simulate the detailed detector response to ground particles. A new energy spectrum in the range 3 $ imes$ 10$^{17}$ eV to 4 $ imes$ 10$^{18}$ eV is presented.

The electron, muon and air- ˇ Cerenkov components of air-showers have been studied simultaneously using the SPASE-2, AMANDA and VULCAN experiments at the South Pole. 32000 events observed by SPASE-2 and VULCAN pass all selection cuts. We report the results of our analysis for these events. The high energy ( 500 GeV) muon content of a subset of these showers will be obtained using the AMANDA-B detector. The combination of muon and information promises a powerful tool for mass composition studies.