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

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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.