Modeling Comptonized Soft Gamma-Ray Flare Emission in Magnetars

中子星是磁星，是宇宙中最有趣的致密天体之一。目前发现的可能有多达30个，其中许多都显示出强大的高能X射线爆发。磁星为测试基础物理提供了一个独特的论坛，目前地球实验室还无法获得这一平台。天体物理学家对量子电动力学(QED)理论抱有很大信心，他们经常使用未经验证的高磁场区域理论预测来模拟中子星环境。因为磁星提供了进行这些测试的实验室，所以了解它们的特征对于将它们用作QED物理实验室的代理至关重要。这个项目通过开发计算机模拟来研究磁星表面以上的区域，在这个方向上迈出了重要的一步。这个研究项目提供了一个重要的研究生和本科生培训元素，让学生参与研究和教学。一名或两名博士生将作为这项工作的一个组成部分，其结果将导致博士论文。这些学生将参与本科教学，每年至少在适当的莱斯课程中讲授一堂关于中子星的讲座。一名或多名本科生将被招募参与赖斯大学高级论文研究的较小、自给自足的部分。预计这些学生将在研究小组研讨会上向莱斯的同龄人展示他们的研究成果。继续与休斯顿天文学会建立的关系，PI还将向公众展示这项工作的方方面面和中子星激发场的亮点。用更专业的术语来说，这个项目开发了一个蒙特卡罗模拟来描述磁磁层中的辐射传输，主要是在封闭的场区。该代码将计算X射线与康普顿散射事件中的热、相对论电子之间的相互作用，跟踪它们之间的能量交换，并考虑光子极化的变化。将发展新的磁康普顿截面的物理形式，以解释将主导不透明度的多个回旋共振。其他过程将被模拟，包括光子分裂和磁对创建，并将评估它们对衰减约100keV以上软伽马射线的影响。为了便于衰减计算，将进行磁性双光子对创建截面的新的物理发展。将包括双光子对的创建和偏振签名的生成，这将是提高精度的必要和亮点，增加了建模的诊断潜力。具体地说，捕获偏振信息可以提供解开几何源信息和强场QED物理签名的手段。创建正电子对和康普顿散射的关键物理分析进展将是这项工作的一个特点，并将为天体物理学家在其他问题上使用新的工具。作为一个更广泛的科学因素，该计划的偏振考虑将有助于确定X射线和软伽马射线波段偏振望远镜的议程和动机。与这些目前处于科学兴趣前沿的源有关的三个悬而未决的问题将受到该项目的重大影响。这些是(I)磁星是否本质上不同于普通脉冲星，或者它们的发射是由在中子星参数空间的不同部分运行的相似物理驱动的；(Ii)是什么导致耀斑的最大能量通常被校准到大约100-300kev；以及(Iii)在磁层中孕育非热辐射的能量注入地点到底在哪里，即，这个地点离恒星表面和/或磁赤道有多近？

Neutron stars that are magnetars are some of the most interesting compact objects in the Universe. There are perhaps as many as 30 presently identified with many of them exhibiting powerful outbursts of high energy X-rays. Magnetars present a unique forum for testing fundamental physics that is not presently accessible in terrestrial laboratories. Having great faith in the theory of quantum electrodynamics (QED), astrophysicists routinely model neutron star environments by using untested theoretical predictions for high magnetic field domains. Because magnetars provide the laboratory for performing these tests, understanding their character is central to using them as a proxy QED physics laboratory. This project makes significant steps in this direction by developing computer simulations to study the region above the surface of magnetars.This research project offers a significant element of graduate and undergraduate training with the involvement of students in research and teaching. One or two PhD students will be engaged as an integral part of the work with the results leading to doctoral theses. These students will be involved in undergraduate pedagogy by teaching at least one lecture per year on neutron stars in appropriate Rice courses. One or more undergraduates will be enlisted to work on smaller, self-contained portions of the research for their senior theses at Rice. These students are expected to present their research results to peers at Rice in research group seminars. Continuing an established relationship with the Houston Astronomical Society, the PI will also present aspects of this work and highlights of the exciting field of neutron stars to the general public.In more technical terms, this project develops a Monte Carlo simulation to describe radiative transfer in magnetar magnetospheres, mostly in closed field zones. The code will compute the interaction between X-rays and hot, relativistic electrons in Compton scattering events, tracking the energy exchange between them and accounting for changes in photon polarization. New physics formalism for the magnetic Compton cross section will be developed to account for the multiple cyclotron resonances that will dominate the opacity. Other processes will be modeled including photon splitting and magnetic pair creation, and their effect on attenuating soft gamma-rays above around 100 keV will be assessed. New physics developments of the magnetic two-photon pair creation cross section will be performed to facilitate the attenuation calculations.The creation of pairs will be incorporated and the generation of polarization signatures will be both a necessity for precision and also a highlight, increasing the diagnostic potential of the modeling. Specifically, capturing polarization information may provide the means to disentangle geometrical source information and the signatures of strong-field QED physics. Key physics analysis developments for positron-electron pair creation and Compton scattering will be a feature of the work and will provide new tools for astrophysicists to employ in other problems. As a broader scientific element, the polarization considerations of this program will aid in defining agendas and motivations for polarimetric telescopes in the X-ray and soft gamma-ray bands.Three outstanding questions pertaining to these sources that are at the forefront of scientific interest at the moment will be impacted significantly by this project. These are (i) whether or not magnetars are inherently different from normal pulsars, or is their emission driven by similar physics operating in different portions of neutron star parameter space, (ii) what causes the maximum energies of flares to be generally calibrated to around 100--300 keV; and (iii) where exactly is the site of energy injection in the magnetosphere that seeds non-thermal radiation in magnetars, i.e., how proximate is this locale to the stellar surface and/or the magnetic equator?