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

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