The STACEE-32 Ground Based Gamma-ray Detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment detector in its initial configuration (STACEE-32). STACEE is a new ground-based gamma ray detector using the atmospheric Cherenkov technique. In STACEE, the heliostats of a solar energy research array are used to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous detectors.Comment: 45 pages, 25 figures, Accepted for publication in Nuclear Instruments and Methods

Detection of Gamma Rays with E > 100 MeV from BL Lacertae

Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright American Astronomical SocietyPeer reviewe

Detection of Multi-TeV Emission from Markarian 421

Gamma rays with energies exceeding 5 ± 1.5 TeV have been detected from Markarian 421 using the Whipple Observatory's 10 m γ-ray telescope. These observations employ a new technique: the so-called large zenith-angle technique. Because they are taken at large zenith angles, the observations yield high statistics data on the multi-TeV part of the spectrum and are well suited for examining the question of a possible energy cutoff. Observations taken during high states on 1995 June 20, 21, and 28 show no evidence for a spectral break. These results conflict with a previous interpretation of the Markarian 421 energy spectrum in which a cutoff due to γ-ray absorption in extragalactic space was postulated

Detection of Gamma Rays with E > 300 GeV from Markarian 501

The detection of gamma rays of energy greater than 300 GeV from the BL Lacertae object Mrk 501 demonstrates that extragalactic TeV emission is not unique to Mrk 421. During 66 hr of observations between 1995 March and July we measured an average flux of 8.1 ± 1.4 × 10-12 cm-2 s-1 above 300 GeV, a flux that is only 20% of the average Mrk 421 flux. The new gamma-ray source has not been reported by the Compton Gamma Ray Observatory as an emitter of gamma rays at lower energies. There is evidence for variability on timescales of days