Collaborative Research: Short Gamma-Ray Bursts Arising From Misaligned Structured Jets in the Dawn of Gravitational Wave Astronomy

中子星(NS)合并事件引力波(GWS)和电磁(EM)引力波的探测，标志着多信使天文学进入了一个新时代。微弱的短伽马射线暴(GRB)GRB170817A的探测提供了最确凿的证据，证明NS合并确实是短伽马射线暴的前身。在这一发现之后，完全理解来自伽玛暴的电磁信号的需求变得更加迫切。加州州立大学、萨克拉门托大学、普渡大学和西北大学之间的一项研究合作将调查短伽玛暴以及它们是如何由喷流产生的，即具有几何结构的电离物质的光束流出。当这样的急流相对于从伽玛暴到地球的方向不对齐时，它们被观测到亮度较低，或者相对较弱，就像在GRB170817A上看到的那样。研究人员将使用相对论磁流体力学(MHD)模拟来充分解释目前和未来从GW信号中观测到的EM对应物。这项拟议研究的一个重要组成部分是培训研究生和本科生，鼓励他们开展原创研究项目。研究小组还将通过在即将开放的萨克拉门托州立天文馆举办一系列公开讲座，扩大对科学和公共扫盲的参与。他们还将继续在三个校园开展外展活动，包括公共太阳能观赏派对、吸引6-12年级学生参加动手活动，以及协助高中教育工作者设计动手课堂演示。提案团队将扩大其正确预测，即中子星合并事件最有可能产生的电磁信号将是来自未对准的伽马射线喷气式飞机的微弱伽马射线脉冲。基于对短伽玛暴喷流的初步MHD模拟，研究小组发现，喷流自然会有某种结构，喷流的性质随角度平滑变化，这样发光核心被更慢、更暗的护套包围，这与GRB170817A的观测结果一致。研究人员将在真实的环境气体剖面和真实的初始黑洞条件下对短伽玛暴喷流进行大规模相对论MHD模拟，以确定喷流的结构。通过这些模拟，他们将从发射区跟踪喷流，穿过封闭的环境气体和爆发距离，在那里形成较慢的护套，一直到预计会产生伽马射线信号的透明区域。这项工作还将得到分析研究的补充，即当喷流从限制介质中冲出时，它将被限制介质准直。该团队将计算一架以任意角度指向观察者的喷气式飞机的预期信号，用于广泛的即时发射模型和伴随的长期余辉排放。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The detection of both gravitational waves (GWs) from a neutron star (NS) merger event and its electromagnetic (EM) counterpart marks a new era in multi-messenger astronomy. The detection of the faint short gamma-ray burst (GRB), GRB170817A, the EM counterpart of a gravitational wave event provides the most conclusive evidence that NS mergers are indeed the progenitors of short GRBs. The need to fully understand the EM signals from GRBs is even more pressing after this discovery. A research collaboration between California State University, Sacramento, Purdue University, and Northwestern University will investigate short GRBs and how they are produced by jets, or beamed outflows of ionized matter, which have geometrical structure. When such jets are misaligned with respect to the direction from the GRB to earth, they are observed to be under-luminous, or relatively faint, as was seen for GRB170817A. The researchers will use relativistic magneto-hydrodynamical (MHD) simulations to fully explain current and future observations of EM counterparts from GW signals. An important component of this proposed research is training both graduate and undergraduate students, who will be encouraged to conduct original research projects. The research team will also broaden participation in science and public literacy with a series of public lectures at the soon-to-be opened Sacramento State planetarium. They will also continue outreach efforts at the three campuses, including public solar viewing parties, engaging grade 6-12 students with hands-on activities and assisting high school educators to design hands-on classroom demonstrations.The proposal team will expand on its correct prediction that the most likely electromagnetic signal from a neutron star merger event would be a faint gamma-ray pulse from a misaligned GRB jet. Based on preliminary MHD simulations of a short GRB jet, the team found that a jet is naturally expected to have some structure, where the jet's properties vary smoothly with angle, such that the luminous core is surrounded by a slower, fainter sheath, consistent with observations of GRB170817A. The researchers will carry out large-scale relativistic MHD simulations of short GRB jets in realistic ambient gas profiles and realistic initial black hole conditions in order to determine the structure of the jet. Through these simulations, they will follow the jet from the launching region, through the confining ambient gas and the break out distance, where its slower sheath forms, all the way to the transparency region where the gamma-ray signal is expected to be produced. The work will be complemented by analytical studies of jet collimation by a confining medium as the jet breaks out from it. The team will calculate the expected signal from a jet pointed at an arbitrary angle to the observer for a broad range of prompt emission models and from the accompanying long-term afterglow emission.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.