Imaging Air Cherenkov Technique (IACT)

The detection of very high energy gamma rays is based on the imaging air Cherenkov technique.
An incident high-energy gamma ray interacts high up in the atmosphere and generates an air shower of secondary particles. The number of shower particles reaches a maximum at about 10 km height, and the shower dies out deeper in the atmosphere. Since the shower particles move at essentially the speed of light, they emit Cherenkov light, a faint blue light. 

The Cherenkov light is beamed around the direction of the incident primary particle and illuminates on the ground an area of about 250 m diameter, often referred to as the Cherenkov light pool. For a primary photon at TeV energy (1012 eV), only about 100 photons per m² are seen on the ground. They arrive within a very short time interval, a few nanoseconds.

A telescope located somewhere within the light pool (Figure 1) will "see" the air shower, provided that its mirror area is large enough to collect enough photons. The "effective detection area" of a Cherenkov telescope is therefore given roughly by the area of the Cherenkov light pool, about 50000 m², to be compared with the sub-m² detection area of satellite instruments aiming to detect gamma rays before they interact with the atmosphere.

 

Figure 1

The image obtained with the telescope shows the track of the air shower, which points back to the celestial object where there incident gamma ray originated. The intensity of the image is related to the energy of the gamma ray. The shape of the image can be used to reject unwanted "background", such as showers induced by cosmic ray particles.

With a single telescope providing a single view of a shower, it is difficult to reconstruct the exact geometry of the air shower in space. The achieve this, multiple telescopes are used which view the shower from different points and allow a stereoscopic reconstruction of the shower geometry. ^[2]

MAGIC, HESS and VERITAS are current generation of pointing telescopes which use IACT to observe VHE gamma-ray of the different sources.

Next generation of these telescope would be Cherenkov Telescope Array (CTA) which is planned to start operation in 2016.