Effects of Nuclear Equation of State on Type-I X-ray Bursts: Interpretation of the X-ray Bursts from GS 1826-24

TL;DR

This study investigates how different neutron star equations of state influence Type I X-ray burst characteristics, using relativistic models to compare with observations from GS 1826-24, thereby constraining neutron star properties.

Contribution

It introduces a relativistic stellar-evolution approach to connect neutron star EOS with X-ray burst profiles, providing new constraints on neutron star radii and EOS.

Findings

Larger neutron star radii lead to longer recurrence times and higher peak luminosities.

EOS stiffness correlates with burst peak luminosity, constraining neutron star models.

Neutrino cooling affects nuclear ignition efficiency and burst behavior.

Abstract

Type I X-ray bursts are thermonuclear explosions on the neutron star (NS) surface caused by mass accretion from a companion star. Observations of X-ray bursts provide valuable information on X-ray binary systems, e.g., binary parameters, the chemical composition of accreted matter, and the nuclear equation of state (EOS) of NSs. There have been several theoretical studies to constrain the physics of X-ray bursters. However, they have mainly focused on the burning layers above the solid crust of the NS, which brings up issues of the treatment of NS gravitation and internal energy. In this study, focusing on the microphysics inside NSs, we calculate a series of X-ray bursts using a general-relativistic stellar-evolution code with several NS EOSs. We compare the X-ray-burst models with the burst parameters of a clocked burster associated with GS 1826-24. We find a monotonic correlation between the NS radius and the light-curve profile. A larger radius shows a higher recurrence time and a large peak luminosity. In contrast, the dependence of light curves on the NS mass becomes more complicated, where the neutrino cooling suppresses the efficiency of nuclear ignition. We also constrain the EOS and mass of GS 1826-24, i.e., stiffer EOSs, corresponding to larger NS radii, are not preferred due to too-high peak luminosity. The EOS and the cooling and heating of NSs are important to discuss the theoretical and observational properties of X-ray bursts.