2025ApJ...981...43C

Because of the previously observed stability of the 171 days period, the superorbital modulation of the low-mass X-ray binary 4U 1820-30 was considered a consequence of a third star orbiting around the binary. This study aims to further verify this triple model by testing the stability of the superorbital period using the light curves collected by X-ray sky-monitoring/scanning telescopes from 1987 to 2023. Both power spectral and phase analysis results indicate a significant change in the superorbital period, from 171 to 167 days, over this 36 yr span. The evolution of the superorbital phase suggests that the superorbital period may have experienced an abrupt change between early 2001 and mid 2003 or changed gradually with a period derivative of ${\dot{P}}_{\sup }=(-4.20\pm 0.72)\times 1{0}^{-4}$ day day^–1. We conclude that the superorbital period of 4U 1820-30 was not as stable as anticipated by the triple model, which strongly challenges this hypothesis. Instead, we propose an irradiation-induced mass-transfer instability scenario to explain the superorbital modulation of 4U 1820-30.