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

The Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE) is a new gr ound-based atmospheric Cerenkov telescope for gamma-ray astronomy. STACEE u ses the large mirror area of a solar heliostat facility to achieve a low en ergy threshold. A prototype experiment that uses 32 heliostat mirrors with a total mirror area of similar to 1200 m(2) has been constructed. This prot otype, called STACEE-32, was used to search for high-energy gamma-ray emiss ion from the Crab Nebula and Pulsar. Observations taken between 1998 Novemb er and 1999 February yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75 sigma in 43 hr of on-source ob serving time. No evidence for pulsed emission from the Crab Pulsar was foun d, and the upper limit on the pulsed fraction of the observed excess was le ss 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 energ y threshold of E-th = 190 +/- 60 GeV, and a measured integral flux of I(E > E-th) = (2.2 +/- 0.6 +/- 0.2) x 10(-10) photons cm(-2) s(-1). The observed flux is in agreement with a continuation to lower energies of the power-la w spectrum seen at TeV energies.