CRII: ACI: Unveiling the Origin of the Highest Energy Particles in the Universe with Large-Scale First-Principle Fully-Kinetic Simulations

该项目的长期目标是揭开宇宙中最高能量粒子的起源——所谓的超高能宇宙射线（uhecr）——带电离子，其能量可以超过一个网球的能量。由于能量最高的离子极其罕见，它们的起源仍然难以捉摸。产生UHECR的主要候选者之一是耀变体，这是一类从超大质量黑洞中产生相对论性喷流的星系。然而，在耀变体喷流中加速uhecr的过程尚不完全清楚。大多数模型试图通过拟合观测到的耀变体发射来推断加速粒子的性质。由于有大量的自由参数，模型不受观测值的唯一约束，因此预测能力很小。事实上，目前还没有从第一性原理中建立起可靠的理论，来解释将喷射能量转移到最高能量粒子的机制，因此，对于uhecr是否来自耀变体喷流，也没有明确的答案。这项提案旨在解决这个基本问题，从而服务于国家科学基金会促进科学进步的使命，通过从第一原理研究耀焰喷流中的粒子加速物理学。该计划还将为天文学、计算机科学和物理系的本科生创造研究机会。在这个过程中，学生将接触到一个活跃的研究环境，并在科学的方法训练。将开发一个互动网站，目的是为公众提供一个方便访问这个令人兴奋的多学科研究领域（在计算、物理和天文学之间的接口）的途径。该网站还将为对最新科学发现感兴趣的高中教师提供一个门户网站。该计划将通过自洽地研究耀blazar相对论喷流中粒子加速的物理特性来研究uhecr的起源。这项研究将探索磁重联中的粒子加速，磁重联是一种相反极性的磁力线湮灭的过程，将它们的能量释放给粒子。粒子加速的物理性质是高度非线性的——重连接过程影响整个射流动力学，这反过来又改变了通过重连接耗散能量的效率——因此很难用分析工具建模。该项目利用了过去几年计算能力的巨大增长和强大的粒子-细胞（PIC）代码的发展，这些代码可以从第一性原理模拟无碰撞等离子体。粒子加速的研究将通过一套前所未有的大范围的二维和三维PIC模拟来进行，以便结果可以从PIC模拟的微观尺度适当地外推到天体物理加速器的宏观尺度。由于耀变体喷流中的等离子体成分没有得到很好的约束，本项目将研究电子-质子和电子-正电子-质子等离子体中质子加速的效率。一个新的粒子冷却模块将在PIC代码中实现，以自一致地考虑辐射损失对电子和质子动力学的影响（即同步辐射和光强子与射流中辐射场的相互作用）。这对于评估耀变体喷流是否能将离子加速到超高能量具有重要意义。

The long-term objective of this project is to unveil the origin of the highest energy particles in the Universe - the so-called ultra-high-energy cosmic rays (UHECRs) --- charged ions whose energies can exceed the energy of a tennis ball. Since the highest energy ions are extremely rare, their origin still remains elusive. One of the leading candidates for UHECR production are blazars, a class of galaxies with relativistic jets emerging from supermassive black holes. However, the processes that can accelerate UHECRs in blazar jets are not fully understood. Most models attempt to infer the properties of the accelerated particles by fitting the observed emission from blazars. Due to the large number of free parameters, the models are not uniquely constrained by the observations and, therefore, have little predictive power. In fact, there is no reliable theory built from first principles for the mechanism that transfers the jet energy to the highest energy particles, and so there is no definite answer to whether UHECRs originate from blazar jets. This proposal aims to address this fundamental problem - thus, serving NSF's mission to promote the progress of science - by studying the physics of particle acceleration in blazar jets from first principles. The proposed program will also create research opportunities for undergraduate students of Astronomy, Computer Science and Physics departments. In the process, the students will be exposed to an active research environment and trained in the scientific method. An interactive website will be developed with the goal of providing the public an easy access to this exciting field of multidisciplinary research (at the interface between computing, physics and astronomy). The website will also provide a portal for high school teachers interested in updating their lectures with current scientific findings. The proposed project will investigate the origin of UHECRs by studying self-consistently the physics of particle acceleration in the relativistic jets of blazars. The research will explore particle acceleration in magnetic reconnection - a process by which magnetic field lines of opposite polarity annihilate, releasing their energy to the particles. The physics of particle acceleration is highly non-linear - the reconnection process affects the overall jet dynamics, which in turn changes the efficiency of energy dissipation via reconnection - and therefore hard to model with analytical tools. The project takes advantage of the enormous growth of computing power in the last few years and the development of powerful Particle-In-Cell (PIC) codes that can model collisionless plasmas from first principles. The investigation of particle acceleration will be performed via a suite of 2D and 3D PIC simulations in unprecedentedly large domains, so that the results can be properly extrapolated from the microscopic scales of PIC simulations to the macroscopic scales of astrophysical accelerators. As the plasma composition in blazar jets is not well constrained, project will investigate the efficiency of proton acceleration in both electron-proton and electron-positron-proton plasmas. A novel cooling module for the particles will be implemented in the PIC code, to account self-consistently for the effect of radiative losses on the electron and proton dynamics (i.e., synchrotron radiation and photohadronic interactions with radiation fields in the jet). This will be of significant importance to assess whether blazar jets can accelerate ions up to ultra-high energies.