A team of researchers from the Chinese Academy of Sciences’ Institute of Modern Physics, along with their collaborators, have made precise measurements of the masses of several important nuclei using state-of-the-art storage-ring mass spectrometry. Their findings have allowed them to explore X-ray bursts on the surface of neutron stars, providing new insights into the properties of these incredibly dense celestial objects.
Their study, which was recently published in Nature Physics, sheds new light on the nature of Type-I X-ray bursts, which are some of the brightest stellar phenomena observed by space-based telescopes. These explosive events occur on the surface of neutron stars as a result of thermonuclear reactions that take place when hydrogen- and helium-rich matter from a companion star accumulates on the surface of the neutron star for a period of hours or days. Despite lasting only 10 to 100 seconds, these X-ray bursts provide a valuable opportunity to learn more about the unique properties of neutron stars.
The rp-process is responsible for powering Type-I X-ray bursts on neutron stars, and germanium-64 is a crucial nuclide in this nuclear reaction sequence. Its position as a “crossroad” in the nuclear reaction process makes it vital for determining the X-ray flux produced during an X-ray burst. Precise measurements of the masses of other nuclei around germanium-64 are also necessary for understanding neutron stars’ properties.
However, measuring these short-lived nuclei with extreme low production yield has been a significant challenge until now. After over a decade of effort, the researchers from the Storage Ring Nuclear Physics Group at IMP have developed an ultrasensitive mass spectrometry technique called Bρ-defined isochronous mass spectrometry (Bρ-IMS). This method is conducted at the Cooler Storage Ring (CSR) of the Heavy Ion Research Facility in Lanzhou (HIRFL) and can efficiently measure short-lived nuclei while being background-free in the measured spectrum. According to Prof. Wang Meng from IMP, the technique can determine a single nuclide’s mass within a millisecond after production with high precision.
The scientists have successfully conducted precise measurements of the masses of several nuclei, including arsenic-64, arsenic-65, selenium-66, selenium-67 and germanium-63, which have provided insight into the nuclear reaction energy related to germanium-64, a critical waiting-point nucleus. The team used these new mass measurements as inputs for X-ray burst model calculations, which revealed changes in the rapid proton capture nucleosynthesis process path, leading to an increased peak luminosity and a prolonged tail duration of the X-ray burst light curve on the surface of the neutron star.
Furthermore, the comparison of model calculations with observed X-ray bursts from GS 1826-24 led to an increase in the distance from Earth to the burster by 6.5%, and a reduction of the neutron star surface gravitational redshift coefficient by 4.8%, indicating a lower density of the neutron star than expected. Additionally, the composition changes of the rapid proton capture nucleosynthesis process reaction products suggested a higher temperature of the neutron star’s outer shell than previously believed after the X-ray burst.
The study provides significant implications for the investigation of neutron star properties, which is an important frontier topic in physics research. “Our precise nuclear mass measurement has allowed us to obtain a more accurate X-ray burst light curve on the neutron star surface, which has enabled us to establish new constraints on the mass and radius relationship of neutron stars,” commented Prof. Zhang Yuhu from IMP.
