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New Insights into the Superorbital Variation of 4U 1820-30
A recent study led by astronomers from Taiwan’s National Central University has uncovered intriguing variations in the superorbital period of the X-ray binary system 4U 1820-30. The findings, detailed in a research paper published on September 13 on the preprint server arXiv, could enhance our understanding of this complex astrophysical entity.
X-ray binaries consist of a regular star or a white dwarf that transfers material to a dense companion, which is typically a neutron star or black hole. These systems are categorized as low-mass X-ray binaries (LMXBs) or high-mass X-ray binaries (HMXBs) based on the companion star’s mass.
4U 1820-30 is classified as an ultra-compact LMXB situated near the core of the globular cluster NGC 6624. It features a neutron star paired with a mass-losing companion, specifically a helium white dwarf weighing between 0.06 and 0.08 solar masses. The system operates on an orbital cycle of roughly 685 seconds.
Past research on 4U 1820-30 has revealed its ability to not only demonstrate orbital and superhump variations but also to exhibit a superorbital modulation with a significantly longer period—approximately 171.03 days, which has remained stable for decades.
However, the recent work by Yi Chou and his team indicates a crucial change in the superorbital period of this binary. Their analysis, utilizing data from several telescopes dating back to 1987, has discovered that the superorbital cycle has shifted from 171 days to 167 days between 1987 and 2023. This transition may have occurred abruptly between late 2000 and early 2023, or it might have been a gradual change characterized by a period derivative of about -0.000358 days per day.
Previous studies hypothesized that the modulation of the superorbital period was possibly due to the influence of an undetected third companion star. Nevertheless, Chou’s latest results challenge this assumption, prompting consideration of alternative explanations. They suggest that an irradiation-induced mass transfer instability could be the driving force behind this observed behavior.
“The accretion stream is anticipated to arise from a small area surrounding the L1 point on the companion star, where the gravitational field is weak. Consequently, this stream is particularly responsive to X-ray radiation impacting that region,” the authors noted in their paper.
To further explore this intriguing phenomenon, the researchers emphasize the need for additional observations coupled with theoretical studies to substantiate the concept of irradiation-induced mass transfer instability in 4U 1820-30.
More information:
Yi Chou et al, The Puzzling Superorbital Period Variation of the Low-mass X-ray Binary 4U 1820-30, arXiv (2024).
DOI: 10.48550/arxiv.2409.08451
Source
phys.org