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

Abstract

Abstract 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 ∼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 ∼5 and ∼10 yr in the 34 yr light curve. Their X-ray luminosities were (1–4) × 1037 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.