The Development of CTA for the Study of Extreme Extragalactic Particle Acceleration

地球正在遭受来自外太空的带电粒子的轰击。这些粒子被称为“宇宙射线”，并且已经在非常大的能量范围内被观察到。最高能量，或超高能宇宙射线（UHECR）是如此充满活力，他们不能包含在我们的银河系。一个UHECR的能量相当于一个质子中每小时100英里的网球。这些粒子是如何在天体中产生的仍然是一个谜。我们从地球上的实验中知道，将粒子加速到这些能量的一小部分是非常困难的-它需要像欧洲核子研究中心的大型强子对撞机这样的巨大机器，耗资数十亿英镑。然而，皮埃尔俄歇天文台（Pierre Auger Observatory）最近的突破表明，UHECR起源于河外，其到达方向与超大质量黑洞驱动的活动星系的位置相关。不幸的是，由于探测器有限的角分辨率和这些带电粒子在磁场中的偏转，对UHECR的直接测量不足以明确地揭示它们的起源。但一切都没有失去！宇宙射线的加速伴随着甚高能量（VHE）伽马射线的产生。VHE伽马射线是简单的光子，就像星光一样，但能量更高（每一个都拥有一万亿个可见光光子的能量）。与带电的宇宙射线不同，伽马射线直接飞向我们，而不会受到银河系和河外磁场的影响。因此，VHE伽马射线为我们宇宙中一些最极端的区域提供了独特的见解。唯一的问题是，这些异常高能的光子很少，所以卫星探测器根本不够大，无法捕捉到它们。我们需要能从几倍于温布利体育场的地方收集伽马射线的仪器。切伦科夫大气成像望远镜提供的正是这一点。当VHE伽马射线进入地球大气层时，它会产生带电粒子簇射，引起蓝色切伦科夫光的闪烁。每次闪光只持续百万分之一秒，所以我们的眼睛不可能记录下不断闪烁的天空。为了探测闪光，我们在大型光学反射器的焦点处使用光电倍增管摄像机。伽马射线是从带电宇宙射线的巨大背景中提取出来的，方法是在相机的视野中搜索指向天文物体位置的狭窄图像。这种“希拉斯参数化”的方法，来自利兹的迈克尔·希拉斯教授的工作，在1989年首次探测到VHE伽马射线源蟹状星云时使用，此后被全世界采用。当前一代的VHE伽马射线望远镜，如高能立体系统，HESS（在纳米比亚）已经揭示了一个散布着明亮伽马射线源的天空。赫斯已经产生了第一个确凿的证据，表明低能量宇宙射线在我们银河系内的超新星遗迹壳中被加速，但福尔斯的灵敏度不够，无法解决UHECR之谜。拟议中的切伦科夫望远镜阵列（CTA）目前正在由一个欧洲天文学家团队设计，将提供HESS的10倍灵敏度，并将在光子天文学的高能前沿运行。我将帮助开发CTA，用于研究银河系外极端粒子加速的特定目的。UHECR研究的CTA优化需要使用尖端技术、数值模拟和HESS数据。到目前为止，伽马射线中的天空有点像从大城市看到的可见光中的天空-只有最亮的物体可见。有了CTA，VHE宇宙的窗口将真正打开，预计将探测到约1000个源。随着宇宙中最高能粒子的适当准备加速器，UHECR将成为其中之一！

The Earth is being bombarded by charged particles from outer space. These particles are known as 'cosmic rays' and have been observed across an astonishingly large range of energies. The highest energy, or ultra-high energy, cosmic rays (UHECRs) are so energetic that they cannot be contained within our galaxy. An UHECR can have an energy equivalent to a 100 mph tennis ball in single proton. How such particles are produced in celestial objects is still a mystery. We know from experiments on Earth that accelerating particles to even a fraction of these energies is very difficult - it requires huge machines such as the LHC, at CERN, costing billions of pounds. However, recent breakthroughs by the Pierre Auger Observatory, built to measure these extreme particles, suggest that UHECRs are extragalactic in origin with arrival directions that correlate with the locations of active galaxies powered by super-massive black holes. Unfortunately, due to the limited angular resolution of the detector and the deflection of these charged particles in magnetic fields, direct measurements of UHECRs are not sufficient to unequivocally reveal their origin. But all is not lost! The acceleration of cosmic rays is accompanied by the production of very high energy (VHE) gamma rays. VHE gamma rays are simply photons, like star-light, but much more energetic (each possesses the energy of a trillion photons of visible light). Unlike charged cosmic rays, gamma rays wing their way directly to us without having their paths bent by galactic and extragalactic magnetic fields. VHE gamma rays therefore offer a unique insight into some of the most extreme regions of our universe. The only trouble is there are very few of these exceptionally energetic photons, so satellite detectors simply aren't big enough to catch them. We need instruments which can collect gamma rays from an area several times the size of Wembley Stadium. Imaging Atmospheric Cherenkov Telescopes offer just that. When a VHE gamma-ray enters the Earth's atmosphere, it generates a shower of charged particles which cause a flash of blue Cherenkov light. Each flash lasts for just ten thousand millionths of a second, so it is impossible for our eyes to register the constantly glittering sky. To detect the flashes we use photomultiplier tube cameras at the focus of large optical reflectors. Gamma rays are extracted from the huge background of charged cosmic rays by searching for narrow images pointing to the position of an astronomical object in the camera's field of view. This method of 'Hillas parameterisation', from the work of Professor Michael Hillas at Leeds, was used in the first detection of a VHE gamma-ray source, the Crab Nebula, in 1989, and has since been adopted world-wide. The current generation of VHE gamma-ray telescopes, such as the High Energy Stereoscopic System, HESS (in Namibia) has revealed a sky scattered with bright gamma-ray sources. HESS. has produced the first hard evidence that lower energy cosmic rays are accelerated in the shells of supernovae remnants within our galaxy, but falls short of the sensitivity required to resolve the mystery of UHECRs. The proposed Cherenkov Telescope Array (CTA), currently under design by a team of European astronomers, will provide 10x the sensitivity of HESS and will operate on high-energy frontier of photon astronomy. I will help to develop CTA for the specific purpose of studying extreme particle acceleration outside our galaxy. The optimisation of CTA for UHECR studies requires the use of cutting-edge technology, numerical simulations and HESS data. So far the sky in gamma rays is a bit like the sky in visible light seen from a big city - only the brightest objects are visible. With CTA the window on the VHE universe will truly be opened and the detection of some 1000 sources is expected. With the appropriate preparation accelerators of the most energetic particles in the Universe, the UHECRs, will be amongst them!