Variability of the symbiotic X-ray binary GX 1+4: Enhanced activity near periastron passage

TL;DR

This study investigates the long-term variability of the symbiotic X-ray binary GX 1+4, revealing enhanced activity near periastron passage and confirming its orbital period through multi-band observations.

Contribution

The paper provides the first detailed analysis of GX 1+4's variability across optical and X-ray bands, highlighting the correlation with orbital phase and clarifying the nature of its quasi-periodic changes.

Findings

Enhanced optical and hard X-ray variability near periastron

Confirmation of the 1161-day orbital period

No periodicity detected in soft X-ray emission

Abstract

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.