A. M. Hillas

An experiment to search for ultra high energy γ-ray sources from the south pole

Abstract We describe the construction and performance characteristics of an extensive air shower array which has been established at the geographic South Pole. The experiment has been designed to search for sources for cosmic rays with primary energies above 50 TeV with an angular resolution of about 1°. The array has an enclosed area of 6235 m 2 and is at an altitude of 2835 m (695 g cm −2 ). The unique advantage of the site is the circumpolar nature of all candidate sources, including SN1987A, which lie at a constant zenith angle.

Optimizing the design of very high energy gamma ray telescopes

Design strategies for gamma ray telescopes which utilize atmospheric Cherenkov emission to detect astronomical photons with energy ≲ few TeV are considered theoretically, e.g. threshold, field of view, coincidence techniques, filters, altitude of observing station and image selection. The design criterion used was to optimize the signal to noise ratio S√B, or figure of merit Q = S2B, where S is the number of photon triggers and B is the number of background events, including both cosmic rays and sky noise. Photon and hadronic showers were simulated using a large computer program, which samples from integral power law spectra with indices −1.5 and −1.25 (sources), and −1.65 (background). The analysis showed an optimum field of view of ∼1.5° and trigger threshold such that sky noise is ∼2–15% of total background. As this is high, a low noise threshold was also considered but hardly alters the conclusions. Fast coincidences between independent detectors provide a very efficient method of reducing the sky noise, e.g., improvements in Q of ∼4.2 and ∼7.5 for two- and threefold coincidences were obtained with spectral index −1.5. Fast timing to reject off-axis (nuclear) images is of limited use.

Measurement of the angular resolution of an extensive air shower array using a Cherenkov light detector

Abstract We describe a method which has been used to evaluate the pointing accuracy and angular resolution of the South Pole air shower array. It makes use of coincidences between the air shower array and a very simple air Cherenkov detector of small aperture. The alignment of the array is shown to be known to be within ±0.2° for the zenith direction and ±0.5° for the azimuth direction. Additionally the angular resolution has been measured at energies below 200 TeV, lower than those at which the subarray comparison technique can be applied. At higher energies conclusions drawn previously from subarray comparisons are confirmed: the angular resolution, as described by the root mean square uncertainty in zenith angle, of showers produced by primaries of 200 TeV is found to be 0.8° for showers incident at about 20° from the vertical.