Ultra-high-energy gamma-ray astronomy using atmospheric Cerenkov detectors at large zenith angles

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Abstract
By operating at large zenith angles, an atmospheric Cerenkov detector can become a powerful telescope for PeV gamma-ray astronomy, attaining an effective detection area many times larger than any existing (or planned) particle-detector array. The large detection area is essential for observing the transient behaviour of the known PeV sources. These ultra-high-energy gamma rays are of special interest because they presumably arise from interactions of nuclei which have been accelerated near a neutron star or other compact object. Although a detector's counting rate decreases with zenith angle (because the threshold energy increases), detection sensitivity to a faint source will increase if the source spectrum is sufficiently flat. Cygnus X-3 is an example. A very sensitive Cerenkov detector can attain the enormous detection area associated with observations with a large zenith angle and still have a threshold energy below the extragalactic cut-off at 100 TeV.