A Prototype of the Cherenkov Telescope Array Achieves First Light with Cosmic-Ray Air Showers

2015-12-09

Institute for Space-Earth Environmental Research (ISEE) is involved in the Cherenkov Telescope Array (CTA), which is a future international observatory for very-high-energy gamma-ray astronomy. CTA will deploy 70 small-sized telescopes with a mirror diameter of about 4 m in the Republic of Chile in order to increase the gamma-ray detection sensitivity in the energy domain beyond 100 tera electronvolts (100 × 10¹² eV), which was difficult to be covered by current gamma-ray telescopes. The Cosmic-Ray Research Devision of ISEE has been developing the focal plane camera of these telescopes.
 A prototype of Gamma-ray Cherenkov Telescope (GCT), which is one of CTA telescope designs, was inaugurated at the Paris Observatory, Meudon, France on December 1, 2015 (Fig. 1). The GCT prototype consists of 6 segmented primary mirrors, a monolithic secondary mirror, and a focal-plane camera with 2048 image pixels (Fig. 2).
 The prototype camera recorded cosmic-ray airshower events during an operation test conducted on November 26 (Fig. 3). These images are CTA’s first ever Cherenkov events.
 When a very-high-energy gamma ray or cosmic ray collides the Earth atmosphere, it yields an “air shower” and transient atmospheric Cherenkov radiation, which is mainly composed of ultraviolet photons. Collecting this Cherenkov radiation by the primary and secondary mirrors of the telescope in Fig. 1, the focal-plane camera can record a very fast “photograph” every 1 nano second (1 billionths second) as shown in Fig. 3.

Figure 1. The GCT prototype installed at the Paris Observatory, Meudon, France.	Figure 2. The GCT camera prototype mounted on the GCT prototype telescope.	Figure 3. CTA’s first cosmic-ray air-shower event recorded by the GCT camera prototype.
Figs 1-2：Akira Okumura (ISEE)! Fig 3：The CTA Consortium