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

有关中子星(NSS)的质量和半径等基本参数的信息对于理解冷致密物质中的核子相互作用非常重要，这也被称为状态方程问题。研究低质量X射线双星中强光球半径膨胀(Pre)X射线爆发的光谱演化对确定NS半径具有重要意义。对NS-NS合并事件GW170817和NETER X射线观测站观测到的毫秒脉冲星脉冲剖面的研究证实了这一结果。用热NS模式大气序列(所谓的冷尾法)对观测到的预暴谱演化进行了拟合。最近，人们考虑了各种系统效应对NS半径测定精度的影响，即爆发后期的快速NS旋转和大气的吸积流加热。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。因为现在已知NS半径具有相对较高的精度(12+/-0.4公里，例如，Al-Mamun等人)。2021年)，这为使用冷却尾部方法研究X射线爆发NSS大气的化学成分提供了可能性。除了发现富氦白矮星作为供体恒星外，还发现了具有明显证据的前X射线爆发，证明大气中有热核灰的丰富。我们计划计算一个新的NS模式大气网格，其中预期核灰的化学成分，并将其用于研究已知的前X射线爆发的光球层化学成分。影响爆发谱演化的第二个重要物理过程是爆发冷却中后期大气与吸积流的相互作用。在预暴的最大值时，吸积停止，可能是由于辐射压力力，并在X射线暴光度下降到爱丁顿光度的一半后明显重新开始。自由落体离子对NS大气的加热作用早有考虑，现在我们计划研究辐射压强以及由此产生的加热大气结构和出射辐射对离子的制动作用。这个问题与研究吸积毫秒X射线脉冲星的辐射面积密切相关，我们也有理由希望用所发展的模型来解释它们的辐射光谱。

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.