Non-standard atmospheres of neutron stars in Low-Mass X-ray Binaries: Thermonuclear ashes after X-ray bursts and interaction with the accretion flow

中子星的质量和半径等基本参数对于理解冷密物质中的核子相互作用（也称为状态方程问题）具有重要意义。对低质量X射线双星中强烈光球半径膨胀（PRE）X射线爆发的光谱演化的研究对确定NS半径做出了重要贡献。这些结果得到了NS-NS合并事件GW 170817和NICER X射线天文台观测到的毫秒脉冲星脉冲轮廓的证实。观测到的PRE爆发的光谱演化与热NS模型大气序列相拟合（所谓的冷却尾方法）。最近，各种系统的影响，对NS半径测定的准确性被认为是，即快速NS旋转和大气的吸积流在后期爆发阶段加热。然而，该方法的两个主要系统不确定性中的第一个是大气化学成分。由于NS半径现在已知具有相对高的精度（例如12+/-0.4km），Al-Mamun等人，2021年），这开辟了使用冷却尾方法研究X射线爆发NS大气化学成分的可能性。除了发现富氦的白色矮星作为供体星外，还发现了具有热核灰烬富集大气的明确证据的PRE X射线爆发。我们计划计算一个新的网格NS模型大气与预期的核灰烬的化学成分，并使用它的调查光球的化学成分已知的PRE X射线爆发。影响X射线暴谱演化的第二个重要物理过程是暴冷却中后期大气与吸积流的相互作用。在PRE爆发的最大值，吸积停止，可能是由于辐射压力，显然在X射线爆发光度下降到爱丁顿光度的一半后重新开始。早先考虑了自由下落离子对NS大气的加热，现在我们计划研究辐射压力对离子的制动以及由此产生的加热大气结构和出射辐射。这个问题与吸积毫秒X射线双星的辐射区域的研究密切相关，我们也有合理的希望用发展的模型来解释它们的辐射谱。

Information about the basic parameters of neutron stars (NSs) such as mass and radius are important for understanding nucleon interaction in cold dense matter, which is also known as the equation-of-state problem. Studies of the spectral evolution of powerful photospheric radius expansion (PRE) X-ray bursts in Low-Mass X-ray Binaries made significant contributions to the determination of NS radii. The results were confirmed by investigations of the NS-NS merger event GW170817 and the millisecond pulsar pulse-profiles observed by the NICER X-ray observatory. The observed spectral evolution of PRE bursts were fitted with sequences of hot NS model atmospheres (the so-called cooling tail method). Recently, the influence of various systematic effects on the accuracy of NS radii determinations were considered, namely, rapid NS rotation and heating of the atmosphere by the accretion flow at late burst stages. However, the first of the two main systematic uncertainties of the method is the atmospheric chemical composition. Since NS radii are now known with a relatively high accuracy (12+/-0.4 km, e.g., Al-Mamun et al. 2021), this opens up the possibility to study the chemical composition of the atmospheres of X-ray bursting NSs using the cooling tail method. Apart from finding helium rich white dwarfs as donor stars, PRE X-ray bursts with clear evidence for enrichment of atmospheres with thermonuclear ashes were discovered. We plan to compute a new grid of NS model atmospheres with chemical composition expected for nuclear ashes, and to use it for the investigation of photospheric chemical compositions of known PRE X-ray bursts. The second important physical process affecting the X-ray burst spectral evolution is the interaction of the atmosphere with the accretion flow at middle and later burst cooling phases. At the maximum of a PRE burst, accretion is stopped, perhaps due to radiation pressure force, and obviously restarts after the X-ray burst luminosity drops to half the Eddington luminosity. The heating of the NS atmospheres with free-falling ions was considered earlier, and now we plan to investigate the braking of the ions by radiation pressure and the arising heated-atmosphere structures and the emergent radiation. This problem is closely connected to the study of the radiation areas in accreting millisecond X-ray pulsars, and we have reasonable hope to explain their radiation spectra using the developed models as well.