Particle Acceleration at Relativistic Shocks: Implications for Models of Gamma-Ray Bursts and Active Galaxies

伽马射线暴（GRBs）是宇宙中已知的最迷人的物体之一，它的独特性体现在它短暂的本质上。大约30年前，它们的发现带来了一系列令人困惑的谜团和惊人的启示。其中最新的进展是在1997年发现的逐渐褪色的x射线、光学和无线电余辉，这些余辉可以精确地测定它们与地球的距离，将它们置于宇宙中最遥远的地方。因此，天体物理学家得出结论：伽马射线暴是宇宙中最明亮、最强大的物体。目前的想法是，它们是由一场灾难性事件产生的，也许是一颗大质量恒星的爆炸。该项目的目标是研究伽马射线暴的灾难性事件模型。在这些模型中，当恒星喷出的物质以极高的超音速和相对论速度进入周围的气体时，就会发生GRB，产生极其强大的宇宙冲击波。能量被释放，既以热的形式，也以极具相对论性的基本粒子的形式。这些高速粒子随后产生辐射，在地球上以伽马射线、x射线、无线电波和可见光的形式被探测到。这种耗散激波情景的不确定性主要来自两个制约因素：(i)瞬发伽马射线数据的持续时间相对较短，能量带宽较窄，以及（ii）目前理论模型的粗糙性。本项目解决了这些限制。它探讨了相对论性粒子如何获得必要的极端能量。它包含了比以前可获得的最大能量更精确的测定。它还试图在粒子加速特性和我们看到的辐射之间建立明确的相关性和趋势，使用后者作为加速物理的诊断探针。这项工作试图回答的大问题包括：(i) GRB环境是否是一个有序的大规模现象，或者它是否拥有大量混乱，纠缠，磁化的细胞，（ii）如果这两种情况都可能出现，从而影响对可能存在两类爆发的观测可能性的解释，而不仅仅是一类，（iii） GRB中的平均磁场有多大，（iv）粒子在爆发中的加速是否足够快，足以解释地球上不断出现的普遍存在的超相对论性原子核（宇宙射线）。伽玛射线爆发的天体物理环境与潜在物理之间的这种联系以前没有被研究过。此外，这项工作还将解决活跃星系的模型，因为它们的发射可能是由类似的物理类型驱动的。这些目标的成功将为我们在未来几十年进一步提高对奇异和诱人的伽马射线暴源的认识提供一个合适的平台。本项目由美国国家科学基金会星系外天文学和宇宙学项目（AST/EXC）资助

AST 0098705BaringGamma Ray Bursts (GRBs) are among the most fascinating objects known in the universe, possessing a uniqueness embodied in their ephemeral nature. Their discovery around 30 years ago has led to a sequence of baffling mysteries and startling revelations. The latest advance among these was the discovery in 1997 of fading X-ray, optical and radio afterglows that have permitted accurate determination of their distance from Earth, placing them in the most remote realms of the universe. Consequently, astrophysicists have arrived at the conclusion that GRBs are the most luminous and powerful objects in the universe. Current thinking is that they are spawned by a catastrophic event, perhaps an explosion of a massive star. The goals of this project are to investigate catastrophic event models of GRBs. In these models, the GRB occurs when the ejected material from the star ploughs into the surrounding ambient gas at highly supersonic and relativistic speeds, creating an enormously powerful cosmic shock wave. Energy is liberated, both in thermal forms and in the form of extremely relativistic elementary particles. These high-speed particles then produce radiative emission, which is detected on Earth as gamma rays, X-rays, radio waves and optical light. The uncertainties in this dissipative shock scenario essentially stem from two constraints: (i) the relatively short duration and narrow energy bandwidth of the prompt gama-ray data, and (ii) the present crudeness of theoretical models. This project addresses these limitations. It explores how the relativistic particles can obtain the necessary extreme energies. It includes much more accurate determinations than previously available of the maximum energies attainable. It also seeks to establish definitive correlations and trends between particle acceleration properties and the radiation we see, using the latter as a diagnostic probe on the acceleration physics. The big questions this work seeks to answer include (i) whether the GRB environment is an ordered large scale phenomenon, or whether it possesses a multitude of chaotic, entangled, magnetized cells, (ii) if both of these situations can arise and thereby effect an explanation of the observational possibility that there might be two classes of bursts, not just one, (iii) how large the mean magnetic field is in a GRB, and (iv) whether particle acceleration in bursts is sufficiently rapid to account for the pervasive population of ultrarelativistic nuclei (cosmic rays) that shower Earth incessantly. This connection between the astrophysical environment of gamma-ray bursts and the underlying physics has not been studied before. In addition, this work will also address models for active galaxies, since their emission is probably driven by similar types of physics. Success in these goals will provide a suitable platform for further advancement of our knowledge of the exotic and alluring gamma-ray burst sources over the coming decades. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***