Our Universe is full of powerful particle accelerators: pulsars, supernova remnants, gamma-ray bursts (GRBs) and AGN. Many experiments were designed to identify these acceleration sites, to understand the acceleration mechanisms and the role that accelerated particles play in star formation and galaxy evolution.
A direct way to identify sources of cosmic particles is to find fluxes of photons (gamma rays – E>105 eV) arriving from a single direction. Such photons are produced by π0 decays which suggest the existence of high-energy hadrons that cause the production of π0 mesons at or near the source. Charged hadrons cannot be used to identify the source because their trajectory is deflected by magnetic fields.
Observation of gamma rays became possible in the 1960s. For the detection of very energetic gamma rays (E>30 GeV), the space-based instruments are not enough. Their flux is too low and detectors with a bigger area located on the ground are necessary. Charged particles moving through the atmosphere with a velocity larger than the local speed of light (the vacuum speed of light divided by the refractive index of the air) produce Cherenkov light. This light is emitted on a narrow cone around the direction of the particle.
From each part of the particle track, the Cherenkov light arrives on a ring on the ground. In an air shower, the initial particle interacts with the atoms in the atmosphere and produce many new particles. The ones which are faster than the local speed of light emit Cherenkov light. At the ground, the imprint of the shower will be the overlapping of the light produced by each particle. Imaging atmospheric Cherenkov telescopes (IACTs) provide the most powerful tool for observing gamma rays and probing the high-energy universe in the TeV (1012 eV) regime.
They have an excellent point source sensitivity and angular resolution, together with a large collection area. Large arrays of imaging telescopes are the natural progression of the IACT technique. Simplistic scaling of the number of telescopes suggests that sensitivity improves with √N (N=number of telescopes).
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