Douglas A. Bramel

STACEE Observations of Markarian 421 during an Extended Gamma-Ray Outburst

The active galaxy Markarian 421 underwent a substantial outburst in early 2001. Between January and May of that year, the STACEE detector was used to observe the source in γ-rays between the energies of 50 and 500 GeV. These observations represent the lowest energy γ-ray detection of this outburst by a ground-based experiment. Here we present results from these observations, which indicate an average integral γ-ray flux of (8.0 ± 0.7 ± 1.5) × 10-10 cm-2 s-1 above 140 GeV. We also present a light curve for Markarian 421 as observed by STACEE from March to May and compare our temporal, as well as spectral, measurements with those of other experiments.

The STACEE ground-based gamma-ray detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) in its complete configuration. STACEE uses the heliostats of a solar energy research facility to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The light is concentrated onto an array of photomultiplier tubes located near the top of a tower. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous ground-based detectors. STACEE is being used to observe pulsars, supernova remnants, active galactic nuclei, and gamma-ray bursts