Study examines unusual behavior of X-ray binary telescope

Astronomers at National Central University in Taiwan have studied an unusual variation in the superorbital period of an X-ray binary known as 4U 1820-30. The results of the study, published September 13 on the preprint server arXivcould help us better understand the nature of this system.

X-ray binaries consist of a normal star or white dwarf transferring mass to a compact neutron star or black hole. Depending on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

4U 1820-30 is an ultra-compact LMXB located near the center of the globular cluster NGC 6624. It consists of a neutron star and a mass-losing companion: a helium-bearing white dwarf with a mass of 0.06 to 0.08 solar masses. The system has an orbital period of about 685 seconds.

Previous observations of 4U 1820-30 have shown that it exhibits not only orbital and superorbital variations, but also a superorbital modulation with a period much longer than the orbital period. This period has been measured to be about 171.03 days and appears to be stable over the past few decades.

However, a team of astronomers led by Yi Chou of National Central University recently reported new findings regarding the modulation of the superorbital period of 4U 1820-30. By analyzing available data from various telescopes since 1987, they detected a significant change in the superorbital period of 4U 1820-30.

According to the study, power spectra obtained with five observing instruments show a significant change in the superorbital period from 171 to 167 days between 1987 and 2023. Further analysis suggests that this period may have undergone an abrupt change between late 2000 and early 2023 or changed smoothly with a period derivative of about -0.000358 days per day.

Previous observations of 4U 1820-30 suggested that the modulation of its superorbital period was caused by a third, undetected companion star in the system. However, the new results obtained by Chou’s team contradict this scenario and suggest other hypotheses. Instead, they suggest that irradiation-induced mass transfer instability could be responsible for this behavior.

“The accretion flow is expected to flow from a small region around the companion’s L1 point, where the effective gravitational field is weak. Therefore, the accretion flow is very sensitive to X-ray irradiation on this region,” the researchers explained.

The authors of the paper added that further observations and theoretical studies are needed to verify this scenario of irradiation-induced mass transfer instability for 4U 1820-30.

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