Collaborative Research: Spectral and Radiation Hydrodynamic Models of Photospheric Radius Expansion X-ray Bursts

中子星(NS)是除黑洞外已知的最致密的星体。当一颗中子星与另一颗星处于双星轨道上，物质从恒星流向中子星时，NS可能会经历爆炸性爆发，其中通常包括X射线的发射。I型X射线爆发是由低质量X射线双星的NS表面上新吸积的富氢和/或氦物质的不稳定热核燃烧提供动力的。大约20%的爆发是光球半径扩展(Pre)爆发，在这种爆发中，爆发的光度如此之高，以至于辐射力驱动一股光学厚度的风，将光球(辐射光线的外壳)抬离NS表面。弗吉尼亚大学(UVA)和德克萨斯大学阿灵顿分校之间的一项研究合作将研究X射线爆发前的物理，使用复杂的计算机模型组合来模拟一维球对称和二维轴对称的X射线爆发，既有牛顿极限，也有广义相对论极限。他们将直接将模拟结果与观测结果进行比较。这项工作将使天文学家更好地了解中子星，它们如何与物质相互作用，以及中子星内部的压力、体积和温度的平衡是什么。该项目还将促进若干小组之间的STEM研究。两名研究生将接受最先进的计算工具培训。PIS将通过一名首席调查员参与退伍军人管理局-北卡罗来纳州联盟和斯佩尔曼学院-UVA交换暑期研究项目，以及另一名参与路易斯斯托克斯少数民族参与暑期研究学院，将研究纳入他们对STEM领域代表性不足群体的本科生研究人员的指导。调查人员还将利用这项研究作为他们公共宣传努力的一部分。拟议的研究将利用最先进的数值模拟工具，结合MESA和雅典娜++代码，提供迄今为止最复杂的预爆发模拟。这包括在爆发的不同点产生合成光谱，以校准通常用于从爆发观测推断中子星质量和半径的假设。这项工作是及时的，因为美国宇航局更好的任务提供了丰富的新观测，为爆发演化的后期阶段提供了更好的光谱覆盖，并确定了爆发光谱中的谱线。光谱模拟将能够表征来自连续辐射的限制，但也有助于解释这些光谱特征。这些将是第一批在完全广义相对论中自洽地解决依赖时间的解决方案的计算之一，同时通过非球形计算探索可能的倾向性依赖。该奖项反映了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。

Neutron stars (NS) are the most compact known stellar objects, apart from black holes. When a neutron star is in a binary orbit with another star, and material flows from the star to the neutron star, the NS can undergo an explosive outburst, which often includes the emission of X rays. Type I X-ray bursts are powered by unstable thermonuclear burning of freshly accreted hydrogen- and/or helium-rich material on the surface of a NS in a low-mass X-ray binary. About 20% of bursts are photospheric radius expansion (PRE) bursts, in which the luminosity of the burst is so high that radiation forces drive an optically thick wind that lifts the photosphere (outer shell from which light is radiated) off the NS surface. A research collaboration between the University of Virginia (UVa) and the University of Texas at Arlington will study the physics of PRE X-ray bursts, using a combination of sophisticated computer models to simulate X-ray bursts in 1D spherical symmetry and 2D axisymmetery, both in the Newtonian and general relativistic limits. They will compare the results of their simulations directly with observations. This work will allow astronomers to better understand neutron stars, how they interact with matter, and what is the balance of pressure, volume and temperature within neutron stars. The project will also facilitate STEM research among a number of groups. Two graduate students will be given training in state-of-the-art computational tools. The PIs will incorporate the research into their mentoring of undergraduate researchers from underrepresented groups in STEM fields through one principle investigator’s participation in the VA-NC alliance and Spelman College-UVa Exchange summer research programs and the other’s participation in the Louis Stokes Alliance for Minority Participation Summer Research Academy. The investigators will also draw on the research as part of their public outreach efforts. The proposed research would leverage state-of-the-art numerical simulation tools, combining MESA and Athena++ codes, to provide the most sophisticated simulation of PRE bursts to date. This includes generation of synthetic spectra at different points in the burst to calibrate assumptions commonly used to infer neutron star masses and radii from burst observations. This work is timely due to the wealth of new observations from NASA’s NICER mission, which provide better spectral coverage for later stages of burst evolution and have identified spectral lines in the burst spectra. Spectral modelling will be able to characterize constraints from continuum emission but also aid in the interpretation of these spectral features. These would be among the first calculations to self-consistently solve for the time-dependent solution in full general relativity while exploring the possible inclination dependence through non-spherical calculations.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.