A search for high-energy gamma-rays from AGN ON+231 with the STACEE-32 detector

The Solar Tower Atmospheric Čerenkov Effect Experiment (STACEE) is a ground-based atmospheric Čerenkov telescope constructed to conduct γ-ray astronomy. The STACEE telescope employs the mirrors of the National Solar Thermal Test Facility (NSTTF) to observe γ-rays at a lower energy threshold than the current generation of imaging atmospheric Čerenkov telescopes. A prototype of the telescope was constructed in 1998 which used 32 heliostat mirrors of the NSTTF with a total collection area of ∼1200 m 2. This prototype called STACEE-32, was used to search for high energy γ-ray emissions from the active galactic nucleus (AGN) ON + 231 (also known as W Coma and 1219 + 285). The STACEE-32 response as a function of energy was calculated for this source and we report a spectral energy threshold of E _th = 130 ± 50GeV. The analysis of data collected in the spring of 1999 resulted in a non-detection of the AGN at this energy. We derived a 95% confidence level upper limit on the integrated γ-ray flux for this source of Φ_int(E > 130 ± 50GeV) ≤ 2.4 × 10−10 cm−2sec −1. This upper limit further constrains the high energy flux extrapolations of this AGN.