Water Cherenkov technology in gamma-ray astrophysics

Abstract

Traditional extensive air shower (EAS) arrays consisted of a sparse array of plastic scintillation counters dispersed over a large area. Typically 1–2% of the enclosed detector area was sensitive to the passage of electromagnetic particles. The resulting telescopes had energy thresholds above 100TeV and did not detect any astrophysical sources of gamma rays. The advent of water Cherenkov technology allowed for the construction of an EAS array that was fully active over 100% of the enclosed area. This lead to the dramatic reduction of the energy threshold of such instruments (near 1TeV) and the subsequent detection of sources of gamma rays. The Milagro detector in the Jemez Mountains of northern New Mexico was the first such instrument built. Milagro discovered the Galactic diffuse emission at 10TeV and at least 3 new sources of TeV gamma rays. These observations have established the role of all-sky instruments in the Tera-Volt energy band. These instruments are uniquely sensitive to transient phenomena, such as gamma-ray bursts and flaring active galaxies. A next generation water Cherenkov detector, HAWC (for High Altitude Water Cherenkov) has now been proposed. In this paper I will discuss the water Cherenkov technique and its use in gamma-ray astrophysics. I will touch upon the physics observations that an instrument such as HAWC can enable and elaborate on the technological advances needed to support further work in this area.