The Puzzling Superorbital Period Variation of the Low-mass X-ray Binary 4U 1820-30

TL;DR

This study analyzes 36 years of X-ray data from 4U 1820-30, revealing significant superorbital period changes that challenge the triple star model and suggest an alternative instability mechanism.

Contribution

It provides the first long-term analysis showing superorbital period variability, questioning the triple star hypothesis for 4U 1820-30.

Findings

Superorbital period changed from 171 to 167 days.

Period experienced an abrupt change or gradual evolution.

Results challenge the triple star model, favoring an instability scenario.

Abstract

Because of the previously observed stability of the 171-day 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 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 days to 167 days over this 36-year 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 $\dot P_{sup}=(-4.20 \pm 0.72) \times 10^{-4}$ day/day. 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.