High‐Energy Gamma‐Ray Observations of the Crab Nebula and Pulsar with the Solar Tower Atmospheric Cerenkov Effect Experiment

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a newground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEEuses the large mirror area of a solar heliostat facility to achieve a lowenergy threshold. A prototype experiment which uses 32 heliostat mirrors with atotal mirror area of ~ 1200\unit{m^2} has been constructed. This prototype,called STACEE-32, was used to search for high energy gamma-ray emission fromthe Crab Nebula and Pulsar. Observations taken between November 1998 andFebruary 1999 yield a strong statistical excess of gamma-like events from theCrab, with a significance of $+6.75\sigma$ in 43 hours of on-source observingtime. No evidence for pulsed emission from the Crab Pulsar was found, and theupper 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 fluxof gamma rays from the Crab. We report an energy threshold of E_{th} = 190 \pm60\unit{GeV}, and a measured integral flux of I (E > E_{th}) = (2.2 \pm 0.6 \pm0.2) \times 10^{-10}\unit{photons cm^{-2} s^{-1}}. The observed flux is inagreement with a continuation to lower energies of the power law spectrum seenat TeV energies.Comment: 27 pages, 8 figures, submitted to Ap