Astronomers have uncovered an intriguing change in the superorbital period of 4U 1820-30, an X-ray binary system, sparking new questions about the nature of this cosmic phenomenon.
Unraveling the Puzzle
Astronomers from the National Central University in Taiwan have delved into the unusual superorbital period variation of the X-ray binary 4U 1820-30. This system, located near the center of the globular cluster NGC 6624, consists of a neutron star and a helium white dwarf companion.
Previous observations had revealed that 4U 1820-30 exhibits not only orbital and superhump variations, but also a superorbital modulation with a period much longer than the orbital period. This superorbital period was measured to be around 171.03 days and appeared to be stable over the last few decades.
However, the new findings reported in this study suggest a significant change in the superorbital period of 4U 1820-30. By analyzing data from various telescopes since 1987, the researchers detected a shift in the superorbital period from 171 to 167 days between 1987 and 2023. This abrupt change or a gradual period derivative of about -0.000358 days per day has raised intriguing questions about the underlying mechanisms driving this unusual behavior.
Exploring Potential Explanations
Previous theories suggested that the superorbital period modulation in 4U 1820-30 might be caused by an undetected third companion star in the system. However, the new results obtained by the research team led by Yi Chou have cast doubt on this scenario.
Instead, the researchers propose an alternative hypothesis: the irradiation-induced mass transfer instability. They explain that the accretion stream in 4U 1820-30 is highly sensitive to the X-ray irradiation onto the region around the L1 point (the point where the gravitational forces of the two bodies are balanced) on the companion star.
This irradiation-induced mass transfer instability could be responsible for the observed changes in the superorbital period. The authors note that the accretion stream is expected to flow from a small region around the L1 point, where the effective gravitational field is weak, making it susceptible to the effects of X-ray irradiation.
However, the researchers acknowledge that more observations and theoretical studies are needed to verify this proposed irradiation-induced mass transfer instability scenario for 4U 1820-30. Understanding the underlying mechanisms behind the superorbital period variation could provide valuable insights into the nature of this intriguing X-ray binary system.
Conclusion
The study of the unusual superorbital period variation in the X-ray binary 4U 1820-30 has opened up new avenues of exploration. The observed shift in the superorbital period challenges previous explanations and points to a more complex interplay between the system’s components. Further observations and theoretical investigations are needed to unravel the mysteries surrounding this cosmic phenomenon, ultimately expanding our understanding of the dynamic nature of X-ray binaries.
