Proceedings of the 29th International Cosmic Ray Conference. August 3-10, 2005, Pune, India

astro-ph/0507449

Abstract:  Cherenkov light from cosmic-ray muons is a significant source of background for the Imaging Atmospheric Cherenkov Technique. However, muon events are also valuable as a diagnostic tool because they produce distinctive ring images, and the expected amount of Cherenkov light per unit arclength can be accurately calculated. We report on a comparison of real and simulated muon events in VERITAS Telescope 1, using this comparison to validate the detector model and to determine the light collection efficiency of the telescope.

Proceedings of the 29th International Cosmic Ray Conference. August 3-10, 2005, Pune, India.

 arXiv:astro-ph/0507445

Abstract:  The Very High Energy Radiation Telescope Array (VERITAS) is a system of four imaging Cherenkov telescopes currently under construction at Kitt Peak, Arizona, USA. The first telescope has been in operation at the Mt. Hopkins basecamp since January 2005. We present here detailed Monte Carlo simulations of the telescope response to extensive air showers. The energy threshold for this stand-alone telescope is calculated to be 150 GeV at trigger level, the gamma-ray trigger rate is 22 gamma's/min. Image parameter distributions, and the quality of gamma-hadron discrimination are calculated and show good agreement with distributions from observations of background cosmic rays and high-energy gamma-rays from the Crab Nebula and Markarian 421. The energy spectrum of the Crab is reconstructed as $(3.26\pm0.9)\cdot 10^{-7}\cdot E^{-(2.6\pm 0.3)}$ m$^{-2}$s$^{-1}$TeV$^{-1}$.

 Proceedings of the 29th International Cosmic Ray Conference. August 3-10, 2005, Pune, India.

astro-ph/0507441

Abstract:  The first VERITAS (Very Energetic Radiation Imaging Telescope Array System) telescope has been in operation at the basecamp of the Whipple Observatory since January 2005. Here we present initial observations of AGN made using this telescope. Although this is engineering data, significant detections of Markarian 421 and Markarian 501 have been achieved.

The Astrophysical Journal, Volume 628, Issue 2, pp. 617-628.

astro-ph/0504397

The direct measurement of the extragalactic background light (EBL) is difficult at optical to infrared wavelengths because of the strong foreground radiation originating in the solar system. Very high energy (VHE; E>100 GeV) gamma rays interact with EBL photons of these wavelengths through pair production. In this work, the available VHE spectra from six blazars are used to place upper limits on the EBL. These blazars have been detected over a range of redshifts, and a steepening of the spectral index is observed with increasing source distance. This can be interpreted as absorption by the EBL. In general, knowledge of the intrinsic source spectrum is necessary to determine the density of the intervening EBL. Motivated by the observed spectral steepening with redshift, upper limits on the EBL are derived by assuming that the intrinsic spectra of the six blazars are ∝E-1.8. Upper limits are then placed on the EBL flux at discrete energies without assuming a specific spectral shape for the EBL. This is an advantage over other methods since the EBL spectrum is uncertain.