Decades' long-term variations in NS-LMXBs observed with MAXI/GSC, RXTE/ASM and Ginga/ASM

TL;DR

This study analyzes over 26 years of X-ray data from MAXI/GSC, RXTE/ASM, and Ginga/ASM to identify long-term sinusoidal variations in neutron star low-mass X-ray binaries, suggesting irradiation-induced mass transfer cycles as a possible cause.

Contribution

It provides the first comprehensive long-term analysis of X-ray variability in NS-LMXBs, revealing sinusoidal patterns and proposing a potential mechanism for these variations.

Findings

Two sources show ~5 and ~10 year sinusoidal variations.

Seven additional sources exhibit similar sinusoidal variability.

Long-term variations may be caused by irradiation-induced mass transfer cycles.

Abstract

We investigated the decades' long-term X-ray variations in bright low-mass X-ray binaries containing a neutron star (NS-LMXB). The light curves of MAXI/GSC and RXTE/ASM covers $\sim$ 26 yr, and high-quality X-ray light curves are obtained from 33 NS-LMXBs. Among them, together with Ginga/ASM, two sources (GX 3$+$1 and GX 9$+$1) showed an apparent sinusoidal variation with the period of $\sim 5$ yr and $\sim 10$ yr in the 34 yr light curve. Their X-ray luminosities were $(1-4)\times10^{37}$ erg s$^{-1}$ in the middle of the luminosity distribution of the NS-LMXB. Other seven sources (Ser X-1, 4U 1735--444, GX 9$+$9, 4U 1746$-$37, 4U 1708$-$40, 4U 1822$-$000, and 1A 1246$-$588) have also similar sinusoidal variation, although the profiles (amplitude, period, and phase) are variable. Compering the 21 sources with known orbital periods, a possible cause of the long-term sinusoidal variation might be the mass transfer cycles induced by the irradiation to the donor star.