Particle acceleration in our galaxy studied with H.E.S.S.

我们对宇宙的大部分知识来自对来自天体的电磁辐射的观测--星光就是这种辐射最明显的例子。现代光学望远镜(例如哈勃太空望远镜)产生的图像为天文学家提供了丰富的信息。然而，这种可见的星光只是入射到地球上的辐射光谱的一小部分。现代天体物理学探索了从无线电波到伽马射线的巨大频率范围，每个频段都提供了关于我们自己和遥远星系的独特信息。到目前为止观测到的最高频率(因此也是最高能量)的辐射是超高能(VHE)伽马辐射。电磁光谱的这一部分在过去十年里才开放给观测(相比之下，光学天文学只有几百年)，但它已经为我们提供了关于天空中一些最令人兴奋的天体的独特信息。一条Vhe伽马射线的能量相当于1万亿个正常可见光的光子。这种极端频率的光只能由非常高能量的粒子产生--例如电子或质子。我们从地球上的实验中知道，将粒子加速到这样的能量是非常困难的--它需要巨大的机器，如欧洲核子研究中心的LEP对撞机，耗资数十亿英镑。这些粒子是如何在天体中产生的仍然是一个谜。我们知道我们的银河系充满了所谓的宇宙射线--高能(实际上是高度相对论的)质子和原子核，但它们的起源尚不清楚。正是为了研究这种宇宙辐射的来源，并提高我们对高能宇宙的理解，现在正在建造几个新的伽马射线天文台。最近建成的高能立体系统(H.E.S.S.)(位于纳米比亚的霍马斯高地)使用尖端技术，用非常高能的伽马射线对天空进行了迄今为止最好的观测。使用这个新的天文台，我希望研究我们银河系中一些最猛烈的地方--粒子被加速到巨大能量的地方。对这种极端地方的观察不仅教会了我们关于物体本身的知识，而且还提供了对极端条件下的自然规律的洞察。普通恒星不能产生伽马射线，但质量足够大的恒星可能会在生命结束时以超新星的形式爆炸。似乎就是在这样的超新星中，大多数宇宙射线都被加速了，但即使经过几十年的研究，我们仍然不确定。我们希望通过使用H.E.S.S.来纠正这种情况。超新星爆炸留下了爆炸恒星的死核，要么是作为超高密度中子星，要么是作为黑洞。快速旋转的中子星(或脉冲星)可能只有1/100秒长的“一天”，并产生高能粒子的“风”。我们也想用H.E.S.望远镜更好地理解这类天体。到目前为止，伽马射线的天空有点像从大城市看到的可见光天空--只有几个明亮的物体可见。在接下来的几年里，随着越来越多的伽马射线源被发现，随着新一代望远镜真正打开高能宇宙的窗户，这种情况将会改变。

Most of our knowledge of the universe comes from the observation of electromagnetic radiation from heavenly objects - starlight is the most obvious example of this radiation. Images generated by modern optical telescopes (for example the Hubble Space Telescope) contain a wealth of information for astronomers. However, this visible starlight is only a tiny fraction of the spectrum of radiation incident on the Earth. Modern astrophysics explores the enormous range of frequencies from radio waves to gamma-rays, with each frequency band providing unique information about our own and distant galaxies. The highest frequency (and hence highest energy) radiation observed so far is very-high-energy (VHE) gamma-radiation. This part of the electromagnetic spectrum has been opened-up to observations for only the last ten years (compared to several hundreds of years for optical astronomy) but is already providing us with unique information on some of the most exciting objects in the sky. A single VHE gamma-ray possesses as much energy as one trillion photons of normal visible light. Such extreme frequencies of light can only be generated by very high energy particles - such as electrons or protons. We know from experiments on Earth that to accelerate particles to such energies is very difficult - it requires huge machines such as the LEP collider at CERN which cost billions of pounds. How such particles are produced in celestial objects is still a mystery. We known that our galaxy is filled with so-called cosmic rays - energetic (indeed highly relativistic) protons and nuclei, but their origin is as yet unknown. It is to investigate the origin of this cosmic radiation, and improve our understanding of the high energy universe, that several new gamma-ray observatories are now being built. The recently completed High Energy Stereoscopic System - H.E.S.S. - (located in the Khomas Highlands of Namibia) uses cutting-edge technology to make the best observations so far of the sky in very high energy gamma-rays. Using this new observatory I hope to study some of the most violent places in our galaxy - places where particles are accelerated to huge energies. Observations of such extreme places teach us not only about the objects themselves but also provide an insight into the laws of nature under extreme conditions. Normal stars cannot produce gamma-rays, but stars that are massive enough can explode at the end of their lives as supernovae. It seems likely that it is in such supernovae that most cosmic rays are accelerated, but even after decades of research, we still do not know for sure. Using H.E.S.S. we hope to remedy this situation. Supernova explosions leave behind the dead core of the star that exploded, either as a super-dense neutron star or a black hole. Fast rotating neutron stars (or pulsars) can have a 'day' that is only 1/100 of a second long and produce a 'wind' of high energy particles. We want to use the H.E.S.S. telescopes to better understand such objects as well. So far the sky in gamma-rays is a bit like the sky in visible light seen from a big city - only a few bright objects are visible. In the next few years this picture will change as more and more sources of gamma-rays are discovered and the window on the high energy universe is truly opened by the new generation of telescopes.