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

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 m 2 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 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.