Context. GX 1+4 belongs to a rare class of X-ray binaries with red giant
donors, symbiotic X-ray binaries. The system has a history of complicated
variability on multiple timescales in the optical light and X-rays. The nature
of this variability remains poorly understood. Aims. We study variability of GX
1+4 on long time-scale in X-ray and optical bands. Methods. The presented X-ray
observations are from INTEGRAL Soft Gamma-Ray Imager and RXTE All Sky Monitor.
The optical observations are from INTEGRAL Optical Monitoring Camera. Results.
The variability of GX 1+4 both in optical light and hard X-ray emission (>17
keV) is dominated by ~50-70d quasi-periodic changes. The amplitude of this
variability is highest during the periastron passage, while during the
potential neutron star eclipse the system is always at minimum, which confirms
the 1161d orbital period that has had been proposed for the system based on
radial velocity curve. Neither the quasi-periodic variability or the orbital
period are detected in soft X-ray emission (1.3-12.2 keV), where the binary
shows no apparent periodicity.Comment: 8 pages, 7 figures, accepted for publication in A&
