Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and their collaborators recently measured the masses of several key nuclei with high-precision by employing a state-of-the-art storage-ring mass spectrometry technique. Using the new mass data, they investigated X-ray bursts on the surface of a neutron star, thus deepening the understanding of neutron star properties. The study was published in Nature Physics. 

Neutron stars are considered to be the densest objects besides black holes. Type-I X-ray bursts, among the brightest stellar objects frequently observed in the sky by space-based telescopes, are violent thermonuclear explosions occurring on the surface of neutron stars. 

Due to the strong gravity of the neutron star, hydrogen- and helium-rich matter from a companion star accretes on the surface of a neutron star for hours or days before igniting thermonuclear burning. The explosion lasts for 10 to 100 seconds, causing a bright X-ray burst. These frequent X-ray bursts offer an opportunity to study the properties of neutron stars. 

The bursts are powered by a nuclear reaction sequence, known as the rapid proton capture nucleosynthesis process (rp-process), which involves hundreds of exotic neutron-deficient nuclides. Among them, the waiting-point nuclides, including germanium-64, play a decisive role.  

“Germanium-64, like a crossroad on the path of nuclear reaction processes, is an important congested section encountered when the nuclear reaction proceeds to the medium mass region. The masses of the relevant nuclei are decisive in setting the reaction path and thereby the X-ray flux produced,” explained ZHOU Xu, first author of the paper and a Ph.D. student at IMP. 

Therefore, precision mass measurements of the nuclei around germanium-64 are essential for understanding X-ray bursts and the properties of neutron stars. However, due to extremely low production yield, it has been very challenging to measure the masses of these short-lived nuclei. As a result, few breakthroughs have been seen for many years worldwide. 

After more than ten years of effort, the researchers from the Storage Ring Nuclear Physics Group at IMP have developed a new ultrasensitive mass spectrometry technique at the Cooler Storage Ring (CSR) of the Heavy Ion Research Facility in Lanzhou (HIRFL). This technique, named as Bρ-defined Isochronous Mass Spectrometry (Bρ-IMS), is fast and efficient, thus particularly suitable for measuring short-lived nuclei with extremely low production yields. 

"Our experiment is capable of precisely determining the mass of a single nuclide within a millisecond after its production, and it is essentially background free in the measured spectrum,” said Prof. WANG Meng from IMP. 

The researchers precisely measured the masses of arsenic-64, arsenic-65, selenium-66, selenium-67 and germanium-63. The masses of arsenic-64 and selenium-66 were experimentally measured for the first time, and the mass precision was significantly improved for the others. With the newly measured masses, all nuclear reaction energies related to the waiting point nucleus germanium-64 have been experimentally determined for the first time or the precision of these measurements has been greatly improved compared to old values. 

The researchers then used the new masses as inputs for X-ray burst model calculations. They found that the new data led to changes in the rp-process path. As a result, the X-ray burst light curve from the surface of the neutron star shows an increased peak luminosity and a prolonged tail duration. 

By comparing model calculations with the observed X-ray bursts of GS 1826-24, the researchers found that the distance from Earth to the burster should be increased by 6.5%, and the neutron star surface gravitational redshift coefficient needs to be reduced by 4.8% to match astronomical observations. These results indicate that the density of the neutron star is lower than expected. In addition, the product abundances from the rp-process reveal that the temperature of the outer shell of the neutron star should be higher than generally believed after the X-ray burst. 

"Through precise nuclear mass measurement, we obtained a more accurate X-ray burst light curve on the surface of the neutron star. By comparing it with astronomical observations, we set constraints on the relationship between the mass and radius of the neutron star from a new perspective,” said Prof. ZHANG Yuhu from IMP. 

This work was conducted in collaboration with researchers from GSI Helmholtzzentrum für Schwerionenforschung, Max-Planck-Institut für Kernphysik, Ohio University, Advanced Energy Science and Technology Guangdong Laboratory, Beijing University, Lanzhou University, Beijing Normal University and East China University of Technology.