Contents

• Overview

• Imaging Technique

• Gamma Ray or Cosmic Ray?

Overview

The Earth's atmoshpere is opaque to high energy radiation. Therefore, direct detection requires a satellite based instrument. These detectors require gamma-rays to enter the instrument and be absorbed, so their sensitivity is limited by their size. Production of gammas falls off quickly at higher energies; satellite instruments just can not be made large enough to image high-energy sources. However, ground-based detectors called atmospheric Cherenkov imaging telescopes (ACITs) can use the interaction between the gammas and the Earth's atmosphere to detect high energy photons iin large numbers. This technique opens a new window for astronomy. Our group works with both the Whipple Observatory, a single ACIT, and VERITAS an array of four telescopes and the technological descendant of Whipple. Other ACITs currently in use include HESS in Namibia, MAGIC in the Spanish Canary Islands, and CANGAROO in Australia.

Imaging Technique

When a VHE gamma-ray photon interacts in Earth's atmosphere, it produces an electron-positron pair. These same electrons can also interact with the atmosphere to give off photonsmore gamma-rays. This results in a cascade of electrons and photons. Some of these ultrarelativistic electrons are moving through the atmosphere faster than the speed of light in the same medium. They thus emit Cherenkov light, an electromagnetic shock-wave analogous to the sonic boom emitted by a supersonic jet. This Cherenkov radiation appears as a faint blue pool of light (150 m in radius) of incredibly short duration (a few milliseconds) at ground level. ACITs use large optical reflectors made of many tessellated mirrors to collect this light and reconstruct the source of the original gamma-ray. Sensitivity can be increased even more by using an array of such telescopes spaced tens to hundreds of meters apart.

Gamma Ray or Cosmic Ray?

Though Cherenkov light is also produced by cosmic-ray showers, light from a gamma-ray shower can be discerned by its comparatively smooth shape, compact angular distribution, and through geometric considerations. We can then separate the two types of events finding a particular set of parameter values that includes mostly gamma rays. These values place limits on which images are used in analysis.