Gamma-Ray Burst Afterglows in the Multi-Messenger Era

伽马射线暴（GRBs）是宇宙中最明亮的事件，能够为与周围星际介质相互作用的相对论性喷流提供动力。grb的两个变体与本研究相关；长伽马射线暴是由超大质量恒星的死亡引起的，短伽马射线暴是由中子星合并引起的。由于这些事件产生的引力波，可以对同一物体进行一致的观测。这一点在2017年得到了证明，当时引力波事件GW170817被用于三角测量和研究伽马射线暴GRB170817A，随后被光学望远镜观测到。这种方法允许研究GRB不直接指向地球（离轴），允许完整的射流结构被潜在地观察。目前使用同步辐射的余辉发射模型不能解释观测到的余辉光曲线中的许多特征。此外，在长（GRB190114C）和短（GRB160821B） grb中都检测到了TeV范围内的能量，这表明康普顿上散射也可能发生，并且应该在模型中考虑。对射流的离轴研究也可以更好地研究其结构和发生在射流不同部分的非热加速度。在这个项目中，我将使用几个不同的软件包来模拟伽马射线暴的射流余辉。首先，将使用Ayache在巴斯攻读博士学位期间开发的移动网格流体力学代码GAMMA进行数值射流模拟，以研究结构射流的长期演化。使用ScaleFit模型拟合工具，将对观测数据进行宽带分析，并与这些模拟结果进行比较。其次，由Jimenez在巴斯攻读博士学位时开发的动力学代码Katu，以及python模块afterglow，将用于研究射流结构内发现的各种发射区。结合在一起，将获得一个更完整的理论图像的余辉射流。虽然如果在博士期间没有成功的观测，档案数据是可用的，但希望随着下一次LIGO的运行，以及新的日本引力探测器KAGRA和切伦科夫望远镜阵列（CTA）在2022年下半年上线，新的数据将变得可用，从而允许模型进行测试和改进。在试图使理论模型与观测到的光曲线相一致时，将考虑射流中的激波加速度和康普顿上散射的影响。射流环境的影响也将被研究，考虑射流的边缘，看看它是否可以播下康普顿上散射所必需的光子。这项研究还将通过研究喷流的相似性或差异性来调查短伽马射线暴和长伽马射线暴是否具有可比性。随着博士学位的进展，将发表有关结构射流模拟、TeV排放以及两者的应用以开发和比较模型与观测数据的出版物。这项研究是在天体物理学领域，这是STFC的研究领域之一。

Gamma-ray bursts (GRBs) are the most luminous events in the Universe, and are capable of powering relativistic jets which interact with the surrounding interstellar medium. Two variants of GRBs are relevant for this research; long GRBs, which are driven by the death of supermassive stars, and short GRBs, which are caused by neutron star mergers. Because of gravitational waves generated by such events, coincident observations can be made on the same objects. This was demonstrated in 2017, when the gravitational wave event GW170817 was used to triangulate and study the gamma-ray burst GRB170817A, which was subsequently seen by optical telescopes. This approach allows for the study of GRB which do not point directly at Earth (off-axis), allowing for the full jet structure to be potentially viewed.Current models of afterglow emission which invoke synchrotron radiation do not explain many features which are found in observed afterglow light curves. Furthermore, energies in the range of TeV have been detected in both long (GRB190114C) and short (GRB160821B) GRBs, suggesting that Compton upscattering may also occur and should be accounted for in models. Off-axis studies of the jet also allows for better studies of its structure and the non-thermal acceleration occuring in different parts of the jet.For this project, I will be making use of several different packages to simulate the jet afterglow of GRBs. Firstly, numerical jet simulations using the moving-mesh hydrodynamic code GAMMA, developed by Ayache for his PhD in Bath, will be performed to study the long-term evolution of structured jets. Using the ScaleFit model fitting tool, a broadband analysis of observational data wil then be done and compared to the results of these simulations. Secondly, the kinetic code Katu, developed by Jimenez for his PhD in Bath, along with the python module afterglowpy, will be used to study the various emission zones found within the jet structure. Combined together, a fuller theoretical picture of the afterglow jet will be obtained. While archival data is available if no observations are successful during the PhD, it is hoped with the next LIGO run, along with the new Japanese gravitational detector KAGRA and the Cherenkov Telescope Array (CTA) coming online in the second half of 2022, that new data will be become available, allowing the model to be tested and refined.The shock acceleration in the jet and the impact of Compton upscattering will be considered in trying to reconcile theoretical models with the observed light-curve. The effect of the jet environment will also be studied, with the edge of the jet being considered to see if it could seed the photons necessary for Compton upscattering. This research will also investigate if short and long GRBs are comparable by studying any similarities or differences in the jet. Publications on the structured jet simulations, the TeV emissions and the application of both to develop and compare the model to observational data will be released as the PhD progresses.This research is in astrophysics, which is one of the research areas of the STFC.