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

Abstract We investigate the decades-long X-ray variations in bright low-mass binaries containing a neutron star (NS-LMXBs). The light curves of MAXI/GSC and RXTE/ASM cover ∼26 yr, high-quality are obtained from 33 NS-LMXBs. Among these, together with Ginga/ASM, two sources (GX 3+1 GX 9+1) showed an apparent sinusoidal variation respective periods ∼5 ∼10 yr 34 curve. Their luminosities were (1–4) × 1037 erg s−1 middle NS-LMXB luminosity distribution. Seven other (Ser X-1, 4U 1735−444, 9+9, 1746−37, 1708−40, 1822−000, 1A 1246−588) have similar variations, although profiles (amplitude, period, phase) variable. Compering 21 known orbital periods, one possible cause long-term might be mass-transfer cycles induced by irradiation to donor star.