Two unstable atoms are rewriting neutron star explosions

The results of the study were published in The Astrophysical Journal on December 1.

What Powers Type I X-Ray Bursts

Type I X-ray bursts are intense and recurring thermonuclear explosions seen throughout the galaxy. They typically occur in low-mass X-ray binary systems, where a dense neutron star pulls material from a nearby companion star. As hydrogen and helium build up on the neutron star's surface, unstable nuclear burning ignites, releasing enormous amounts of energy.

This explosive process is driven by rapid proton capture reactions, known as the rp-process. During the rp-process, atomic nuclei quickly absorb protons and transform into heavier elements. How fast these reactions occur, and which nuclear pathways dominate, depends strongly on the exact masses of the nuclei involved.

Why Nuclear Masses Are Hard to Pin Down

Many of the nuclei involved in the rp-process exist near the proton drip line, meaning they are highly unstable and decay very quickly. Because of their short lifetimes, their masses have often been poorly known or completely unmeasured. This lack of data has made it difficult for scientists to accurately model nuclear reactions during X-ray bursts.

According to Dr. Xinliang Yan of IMP, one of the corresponding authors of the study, researchers have debated for years whether a reaction pathway involving phosphorus-26 and sulfur-27 plays a significant role in the rp-process. The uncertainty stemmed largely from missing or imprecise mass measurements for these nuclei.

Measuring Rare Nuclei With High Precision

To resolve this issue, the research team directly measured the masses of phosphorus-26 and sulfur-27 using magnetic-rigidity-defined isochronous mass spectrometry. The experiments were carried out at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou (HIRFL-CSR).

The new measurements revealed that the proton separation energy of sulfur-27 is 129-267 keV higher than earlier estimates. The precision of this value represents an eightfold improvement compared with previous data.

Faster Reactions Inside Exploding Stars

Using the updated mass values, the researchers recalculated how nuclear reactions proceed during X-ray bursts. Under typical burst conditions, they found that the reaction rate of 26P(p,γ)27S increases significantly across temperatures ranging from 0.4-2 Gigakelvin (GK). At 1 GK, the reaction rate can be as much as five times higher than earlier estimates.

The revised data also greatly reduced uncertainty in the reverse reaction rate. As a result, the models predict a higher abundance of sulfur-27 relative to phosphorus-26, indicating that nuclear material flows more efficiently toward sulfur-27 during these stellar explosions.

"Our high-precision mass results and the corresponding new reaction rate provide more reliable input for astrophysical reaction networks, resolving the uncertainties in the nucleosynthesis pathways within the phosphorus-sulfur region of X-ray bursts," said Dr. Suqing Hou from IMP, another corresponding author of the study.

International Collaboration and Research Support

The project was carried out in collaboration with scientists from Germany's GSI Helmholtz Centre for Heavy Ion Research and the Max Planck Institute for Nuclear Physics, along with researchers from Saitama University in Japan.

Funding for the research was provided by the National Key Research and Development Program of China, the Youth Innovation Promotion Association of CAS, and the Regional Development Young Scholars Project of CAS.