How can we build such a detector?

The energy threshold of an ACT telescope scales as sqrt(omega*tau/A_eff) where omega is the angular aperture of the telescope, tau is its resolving time and A_eff is its effective area. omega and tau have lower limits dictated by the physics of the air shower phenomenon but A_eff can be increased significantly given enough resources. At a solar power station the enormous mirror area which can be put to use for astronomy allows one to build a detector with a threshold well below 100 GeV at a modest cost. With 48 heliostats in use, STACEE will have a collecting area in excess of 1800 square meters, approximately 24 times that of the Whipple dish. In principle, this will allow us to attain a threshold under 40 GeV which is close to the upper limit of current and planned satellite detectors (such as GLAST, the next NASA mission, scheduled for 2005 at the earliest). This will allow us to fill in the gap in the spectrum where the EGRET sources disappear.

Projected Sensitivity

Using Monte Carlo programs, together with the results obtained in tests so far we have estimated the sensitivity of the STACEE detector. The results are summarized in figure 4 where STACEE is compared to EGRET and Whipple as well as the MILAGRO detector.

For STACEE and Whipple, we plot the fluxes which would give a 5 sigma signal above background for 50 hours of observation. This is the typical time that can be devoted to a single source during an observing year, given the duty factors involved and the fact that one must spend an equal amount of time observing a neighbouring patch of sky to determine `off-source' background. MILAGRO is a high altitude water Cherenkov detector under construction near Los Alamos, NM. It detects the electrons in air showers directly and can therefore operate with a 100% duty cycle. It does not `point' but reconstructs shower arrival directions from the data. Its curve is 5 sigma sensitivity for one year of observations. The EGRET curve is based on its exposures over its lifetime. As can be seen from the figure, STACEE complements very well existing or near term gamma ray instruments. As a measure of its physics reach, we have plotted two possible curve for the AGN 4C+29. One is a simple power law extrapolation from EGRET measurements and the other assumes a spectral cut-off at 80 GeV. The simulated data points have 2 sigma errors. Of the EGRET sources that pass into the STACEE acceptance, we expect to measure at least 10 at the greater 5 sigma level in the course of one year.

Another way of quantifying STACEE's sensitivity is to report its response to the Crab, a strong and steady source. In one hour on-off (30 minutes each) session, the Crab will result in an 8 sigma excess.