New concept of very high energy cosmic ray observation wide field-of-view telescope

Abstract

The chemical composition of the very high energy cosmic rays (VHECRs) is an important piece of information to investigate their origin and acceleration mechanism. Possible change of chemical composition around the knee energy at ∼ a few times of 1015 eV has been reported by air shower (AS) experiments. So far low flux of VHECRs along with uncertainties due to indirect detection, has limited the precision of chemical composition measurement. It is thus essential to improve the primary mass estimation via the measurement of additional parameters in AS observations. In the present work, we propose a new concept of VHECR observation using ultra-wide fieldof- view (FOV) refractive imaging atmospheric Cherenkov telescopes (IACTs). In the framework of the JEM-EUSO mission, wide FOV optics have been designed for ultra-high energy cosmic ray (UHECR) observation from Space. The pathfinder experiments testing such optics have been successfully operated on balloon and on the ground. Above ∼ 1015 eV, the Cherenkov light from VHECR initiated ASs spreads over a few km, and it can be detected even by meter-scale telescopes. Cherenkov images of distant ASs may span ∼ 20° or more and thus wide FOV optics is necessary for their detection and characterization for chemical composition studies. In addition, we also discuss prospective advantages of EUSO-type wide-FOV telescopes for VHECR and UHECR physics.