宇宙線粒子
私たちの宇宙には、非常に強力な粒子加速器が数多く存在します。代表的なものには、パルサー、超新星残骸、ガンマ線バースト (GRB)、活動銀河核 (AGN)などがあります。
こうした加速領域を特定し、粒子加速の仕組みを理解し、高エネルギー粒子が星形成や銀河進化に果たす役割を明らかにするため、多くの実験が設計されてきました。
宇宙線粒子の発生源を直接特定する方法のひとつは、ある方向から到来する光子 (ガンマ線・E＞10⁵ eV)を観測することです。これらの光子は π⁰ 中間子の崩壊によって生じるもので、光子が観測されるということは、π⁰ 中間子を生成した高エネルギーのハドロンが、その天体付近に存在することを示唆します。一方、荷電ハドロンは磁場によって軌道が曲げられてしまうため、発生源の特定には利用できません。
ガンマ線観測が可能になったのは1960年代のことです。しかし、非常に高エネルギーのガンマ線 (E＞30 GeV)を観測するには、宇宙空間に設置された装置だけでは不十分です。これらのガンマ線はフラックスが極めて小さく、より大きな有感面積を持つ地上観測装置が必要になります。大気中を局所光速 (真空中の光速を大気の屈折率で割った値)よりも速く運動する荷電粒子は、チェレンコフ光を放射します。この光は粒子の進行方向を中心とした狭い円錐状に放射されます。
粒子の飛跡の各点から放たれたチェレンコフ光は、地上では“リング”として到達します。空気シャワーでは、初期の高エネルギー粒子が大気中の原子と相互作用し、多数の2次粒子が生成されます。このうち局所光速より速い粒子はチェレンコフ光を放射します。地上に到達する光の像は、それぞれの粒子が放った光が重なり合ったものとなります。大気チェレンコフ望遠鏡 (IACT: Imaging Atmospheric Cherenkov Telescope)は、TeV (10¹² eV)領域のガンマ線観測において、最も強力な手法を提供します。
IACTは、高い点源感度と優れた角度分解能、そして広い収集面積を備えています。さらに、複数の望遠鏡を組み合わせた大規模アレイは、IACT技術の自然な発展形であり、望遠鏡の数Nに対して、単純なスケーリングでは感度が √N に比例して向上することが示唆されています。
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