High energy gamma-ray observations of the Crab Nebula and pulsar with the Solar Tower Atmospheric Cherenkov Effect Experiment

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cherenkov 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 which 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 November 1998 and February 1999 yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75σ in 43 hours 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 < 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 E_th = 190 ± 60 GeV, and a measured integral flux of I(E > E_th) = (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.