Precision measurements of the core’s mass reveal the nature of a neutron star
(Nanowerk News) 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 using advanced storage ring mass spectrometry techniques. Using the new mass data, they investigated the X-ray bursts on the surface of the neutron star, thereby deepening our understanding of the nature of the neutron star.
The study was published in Natural Physics (Mass measurements show a slowdown in the rapid proton capture process at the nuclear waiting point 64ge”).
Neutron stars are considered as the densest objects other than black holes. I-type X-ray bursts, among the brightest stellar objects frequently observed in the sky by space-based telescopes, are catastrophic thermonuclear explosions that occur on the surface of a neutron star.
Due to the strong gravity of the neutron star, hydrogen and helium-rich material from the companion star accumulates on the surface of the neutron star for hours or days before triggering thermonuclear combustion. The explosions last for 10 to 100 seconds, causing a burst of bright X-rays. These frequent bursts of X-rays offer an opportunity to study the properties of neutron stars.
The bursts are powered by a sequence of nuclear reactions, known as the fast proton capture nucleosynthesis process (rp process), which involves hundreds of exotic neutron-deficient nuclides. Among these, point waiting nuclides, including germanium-64, play a decisive role.
“Germanium-64, like a crossroads in the pathways of nuclear reaction processes, is an important solid state encountered when nuclear reactions proceed into the intermediate mass region. The relevant core mass is decisive in regulating the reaction pathway and thus the X-ray flux is generated,” explained ZHOU Xu, first author of the paper and Ph.D. student at IMP.
Therefore, precise measurements of the core mass around germanium-64 are critical to understanding X-ray bursts and the properties of neutron stars. However, due to very low production yields, it is very difficult to measure the mass of these short-lived nuclei. As a result, several breakthroughs have been seen over the years around the world.
After more than ten years of effort, researchers from the Storage Ring Nuclear Physics Group at IMP have developed a new ultrasensitive mass spectrometry technique in the Cooler Storage Ring (CSR) of the Heavy Ion Research Facility in Lanzhou (HIRFL). This technique, named Bρ-defined Isochronous Mass Spectrometry (Bρ-IMS), is fast and efficient, making it especially suitable for measuring short-lived nuclei with very low yields.
“Our experiments were able to determine precisely the mass of a nuclide within one millisecond of its production, and were 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 measured experimentally for the first time, and the precision of the masses was significantly improved for the others. With the newly measured mass, all the nuclear reaction energies associated with the germanium-64 nuclear waiting point have been determined experimentally for the first time or the precision of these measurements has been greatly improved compared to the old values.
The researchers then used the new mass as input for model calculations of the X-ray bursts. They found that the new data caused a change in the rp process path. As a result, the light curve of the X-ray bursts from the surface of the neutron star shows an increased peak luminosity and an elongated tail duration.
By comparing the model calculations with the observed X-ray bursts from GS 1826-24, the researchers found that the distance from Earth to the bursts must be increased by 6.5%, and the gravitational redshift coefficient of the surface of the neutron star 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 abundance of products from the rp-process reveals that the outer shell temperature of the neutron star should be higher than is commonly believed after an X-ray burst.
“Through precise measurements of the nuclear mass, we obtained a more accurate curve of the light of the X-ray bursts on the surface of the neutron star. By comparing it with astronomical observations, we set limits on the relationship between the mass and radius of a neutron star from a new perspective,” said Prof. ZHANG Yuhu from IMP.
