Newswise — The Chinese Academy of Sciences' Institute of Modern Physics (CAS IMP) and its partners have recently utilized cutting-edge storage-ring mass spectrometry to accurately measure the masses of various crucial nuclei. With the help of this fresh mass data, they studied X-ray bursts occurring on the surface of a neutron star, thereby enhancing comprehension of neutron star characteristics. This research has been published in Nature Physics.
Neutron stars rank among the densest objects after black holes. Type-I X-ray bursts, which are among the most luminous celestial phenomena regularly detected in space using telescopes, involve intense thermonuclear blasts transpiring on the surface of these stars.
The robust gravitational force of neutron stars attracts hydrogen- and helium-dense material from a companion star, which gradually accumulates on the star's surface for several hours or days. Once the matter ignites thermonuclear burning, a powerful explosion ensues, resulting in a brilliant X-ray burst that persists for 10 to 100 seconds. These frequent X-ray bursts provide a valuable opportunity to investigate the characteristics of neutron stars.
The X-ray bursts are fueled by a nuclear reaction sequence called the rapid proton capture nucleosynthesis process (rp-process), which comprises numerous rare neutron-deficient nuclides. The waiting-point nuclides, including germanium-64, are particularly crucial in this process.
"Germanium-64 serves as a significant bottleneck on the route of nuclear reaction processes, representing a crucial congested region as the reaction advances towards the medium mass range. The masses of the pertinent nuclei significantly influence the reaction pathway and, consequently, the resultant X-ray emission," clarified ZHOU Xu, the lead author of the study and a Ph.D. candidate at IMP.
Consequently, accurate mass measurements of the nuclei in the vicinity of germanium-64 are crucial in comprehending the characteristics of X-ray bursts and neutron stars. Nevertheless, owing to the extremely low production yield, determining the masses of these short-lived nuclei has been immensely challenging. Therefore, there have been only a few breakthroughs in this field for many years, both in China and worldwide.
Following over a decade of intensive research, the Storage Ring Nuclear Physics Group at IMP has successfully devised a novel and highly sensitive mass spectrometry technique at the Heavy Ion Research Facility in Lanzhou (HIRFL)'s Cooler Storage Ring (CSR). Known as Bρ-defined Isochronous Mass Spectrometry (Bρ-IMS), this technique is swift and efficient, making it ideal for gauging the masses of short-lived nuclei that possess exceedingly low production yields.
"Our experiment can accurately measure the mass of an individual nuclide within a mere millisecond of its generation, and the spectrum obtained is practically free of background noise," stated Prof. WANG Meng, a member of IMP.
The scientists conducted highly accurate mass measurements of several nuclei, including arsenic-64, arsenic-65, selenium-66, selenium-67, and germanium-63. Arsenic-64 and selenium-66's masses were determined for the first time through this experiment, whereas the accuracy of the measurements for the other nuclei was considerably enhanced. Thanks to these freshly acquired mass values, all the nuclear reaction energies relevant to the germanium-64 waiting point nucleus have been determined experimentally for the first time, or the precision of the existing measurements has been considerably upgraded.
Subsequently, the scientists employed the newly obtained mass measurements as inputs for their X-ray burst model calculations. Their analysis revealed that the updated data led to alterations in the rp-process pathway, causing the X-ray burst light curve emanating from the neutron star's surface to exhibit a heightened peak luminosity and a more extended tail duration.
Upon analyzing the X-ray bursts of GS 1826-24 and comparing them with the model calculations, the team determined that the distance between the burster and Earth ought to be raised by 6.5%. Moreover, they also found that the neutron star surface gravitational redshift coefficient needs to be decreased by 4.8% to be in agreement with astronomical observations. These findings suggest that the neutron star's density is lower than what was anticipated. Additionally, the product abundances obtained from the rp-process indicate that the temperature of the neutron star's outer shell should be higher than what was previously assumed following the X-ray burst.
Prof. ZHANG Yuhu from IMP commented, "With the help of precise nuclear mass measurement, we were able to derive a more accurate X-ray burst light curve on the surface of the neutron star. By comparing it with astronomical observations, we were able to establish new constraints on the relationship between the mass and radius of the neutron star, which provides a fresh perspective for future studies."
This study was carried out in cooperation with scientists 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.
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