STACEE

Abstract: We present the analysis and results of recent high-energy gamma-ray observations of the BL Lac object 3C 66A conducted with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). During the 2003-2004 observing season, STACEE extensively observed 3C 66A as part of a multiwavelength campaign on the source. A total of 33.7 hours of data was taken on the source, plus an equivalent-duration background observation. After cleaning the data set a total of 16.3 hours of live time remained, and a net on-source excess of 1134 events was seen against a background of 231742 events. At a significance of 2.2 standard deviations this excess is insufficient to claim a detection of 3C 66A, but is used to establish flux upper limits for the source.

Abstract: The Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cerenkov telescope for gamma-ray astronomy. STACEE uses the large mirror area of a solar heliostat facility to achieve a low energy threshold. A prototype experiment that uses 32 heliostat mirrors with a total mirror area of ~1200 m2 has been constructed. This prototype, called STACEE-32, was used to search for high-energy gamma-ray emission from the Crab Nebula and Pulsar. Observations taken between 1998 November and 1999 February yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75 σ in 43 hr of on-source observing time. No evidence for pulsed emission from the Crab Pulsar was found, and the upper limit on the pulsed fraction of the observed excess was less than 5.5% at the 90% confidence level. A subset of the data was used to determine the integral flux of gamma rays from the Crab. We report an energy threshold of Eth = 190 ± 60 GeV, and a measured integral flux of I(E > Eth) = (2.2 ± 0.6 ± 0.2) × 10-10 photons cm-2 s-1. The observed flux is in agreement with a continuation to lower energies of the power-law spectrum seen at TeV energies.

Abstract: There are currently no experiments, either satellite or ground based, that are sensitive to astrophysical γ-rays in the energy range between 20 and 250 GeV. We are developing the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) to explore this energy range. STACEE will use heliostat mirrors at a solar research facility to collect Cherenkov light in extensive air showers produced by high-energy γ-rays. Here we report on the results of on-site test work at the solar facility. We demonstrate that the facility is suitable for use as an astrophysical observatory, and using a prototype of STACEE consisting of a subset of the mirrors to be used for the whole array, we detect atmospheric Cherenkov radiation at energies lower than any other experiment to date. Based upon these results we are confident that the eventual STACEE instrument will be capable of exploring the γ-ray sky between 50 and 500 GeV with good sensitivity.

Abstract: We have used the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) high-energy gamma-ray detector to look for fast blue-green laser pulses from the vicinity of 187 stars. The ST...