Lowering the Gamma Ray Energy Threshold at Th\a'emis
Session 16 -- >100 GeV Gamma Ray Astronomy
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [16.02] Lowering the Gamma Ray Energy Threshold at Th\a'emis

David A. Smith (LPNHE -- Universit\a'e de Paris VI-VII), Patrick Fleury (LPNHE -- Ecole Polytechnique), Eric Par\a'e (LPNHE -- Ecole Polytechnique), Jean Quebert (LPT -- Universit\a'e de Bordeaux)

The power-law spectra of the $0.1 < E_\gamma < 30$ GeV point sources detected by EGRET on the Compton GRO have softened at the energies of the atmospheric Cherenkov telescopes ($E_\gamma > 300$ GeV), to the point that only one AGN and two pulsars are visible with current ground-based instruments. Measurement of the spectra in the roll-over range probes details of pulsar and AGN acceleration models, and probes extragalactic infrared photon densities through the absorption of gamma rays. Measurement requires lowering the ground-based energy thresholds to the Egret energy range, and several groups are studying the design of a very large area mirror or mirror array (few thousand M$^2$). Solar power plants built in the 1980's to focus sunlight on boilers in central receiver towers may provide the basic instrument at low cost. This talk describes efforts to use the solar farm at Th\a'emis in the French Pyrenees for a proof-of-principle prototype for a large area Cherenkov telescope. (Similar feasibility studies are underway at the Solar One site in southern California.) Simulations show that cosmic ray backgrounds mostly vanish below 50 GeV. Electron backgrounds can be reduced by optimizing angular resolution. Excellent flux sensitivity should be obtainable. Secondary optics and electronic timing corrections have been studied and tests to measure Cherenkov pulses will be made this winter. The tests will complement data from the ASGAT and THEMISTOCLE experiments and from the CAT imaging telescope now under construction at Th\a'emis.