X-ray bursts are thermonuclear explosions on neutron-star surfaces fueled by hydrogen and helium, with nucleosynthesis paths governed by rapid proton capture (rp-process) sensitive to precise nuclear masses.

The measurements used magnetic-rigidity-defined isochronous mass spectrometry at the Cooling Storage Ring of HIRFL-CSR in Lanzhou, yielding a sulfur-27 proton separation energy more precisely by about 129–267 keV with eightfold improved precision.

Funding for the work came from China’s National Key Research and Development Program, the CAS Youth Innovation Promotion Association, and the Regional Development Young Scholars Project.

By reducing pathway uncertainties, the work strengthens the modeling of phosphorus-sulfur nucleosynthesis within Type I X-ray bursts.

Updated mass data substantially raise the 26P(p,γ)27S reaction rate under X-ray burst conditions (0.4–2 GK), up to five times previous estimates at 1 GK, and greatly reduce uncertainties in the reverse rate.

The study was published in The Astrophysical Journal on December 1 and involved international collaboration with Germany’s GSI, the Max Planck Institute for Nuclear Physics, and Japan’s Saitama University.

Collaborative contributions came from researchers at GSI, the Max Planck Institute for Nuclear Physics, and Saitama University in Japan.

In the rp-process under studied conditions, the revised masses suggest an enhanced pathway toward sulfur-27, increasing the reaction flow.

The findings are documented in The Astrophysical Journal (2025) with the article titled Precision Mass Measurement of 26P and 27S and Their Impact on the 26P(p,γ)27S Reaction in Stellar X-Ray Bursts.

The improved mass data offer more reliable input for astrophysical reaction networks and help resolve uncertainties in nucleosynthesis pathways in the phosphorus-sulfur region during X-ray bursts.

A team from the Institute of Modern Physics (CAS) directly measured the masses of phosphorus-26 and sulfur-27 using magnetic-rigidity-defined isochronous mass spectrometry at HIRFL-CSR in Lanzhou, providing crucial data for the rp-process in Type I X-ray bursts.

This measurement directly informs the nuclear reaction rate calculations during X-ray bursts by narrowing uncertainties in the phosphorus-sulfur region.

The updated mass data indicate a higher sulfur-27 to phosphorus-26 abundance ratio, implying a more efficient flow toward sulfur-27 in the rp-process across the relevant temperatures.