New arsenic-65 proton capture rate changes thermonuclear X-Ray bursts

(ORDO NEWS) — A research team from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), together with international colleagues from Monash University and the Joint Institute for Nuclear Astrophysics, has calculated a significantly revised rate of the arsenic-65 proton capture reaction for the extreme astrophysical conditions of accreting neutron stars, allowing astrophysicists to investigate the mechanism periodic thermonuclear X-ray bursts. This study was published in The Astrophysical Journal.

An extreme astrophysical environment exists in the extremely dense shell of a neutron star, which receives stellar fuel from a companion star. The density of such a shell can be about 6,600 times higher than the density of the core of the Sun, and the temperature – 130 times.

In such extreme conditions, a thermonuclear explosion can occur. Light nuclei fuse into heavier nuclei, and then heavy nuclei capture additional protons and alpha particles. A nuclear explosion releases a huge amount of energy.

Shortly after a thermonuclear explosion, a burst of high-energy X-rays is emitted from the surface. This can be observed in the form of so-called Type I X-ray bursts. As accretion continues, such thermonuclear bursts can repeat periodically. One of the most famous examples is a periodic X-ray burst called GS 1826-24.

During a fusion burst, the arsenic-65 and selenium-66 isotopes are synthesized by subsequent proton capture on germanium-64.

In this study, the scientists re-evaluated the rate of the proton capture reaction on the arsenic-65 isotope under conditions consistent with the extremely high temperature environment of Type I X-ray bursts.

They used a new and more accurate proton threshold for arsenic-65, which was derived from the relativistic Hartree-Bogolyubov theory with density-dependent meson-nucleon coupling.

This new reaction rate changes the nucleosynthesis pathway and the rate at which fusion combustion can occur. In turn, this affects the brightness and temporal variations of type I X-ray bursts, in particular, for the late period, in which nuclear reactions on heavy nuclei dominate.

It also leads to changes in the “ash” of the burst, the nuclei synthesized from X-ray bursts. This updated, more accurate reaction rate refines and deepens our understanding of the hydrodynamics of periodic Type I X-ray bursts.

Moreover, these results critically affect the derived ratio of the mass and radius of a neutron star, which, in turn, imposes restrictions on the equation of state for high-density nuclei.

The new dependence of the mass of the neutron star GS 1826-24 on radius indicates that its stellar mass and radius may be in the same range as that of the pulsar PSR J1903+0327. Information about the properties of neutron stars influences the use of these properties in gravitational wave astronomy.

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