Unveiling the intensity-dependent wake structure of Vela X–1 using MAXI/GSC

Context. Vela X−1 is one of the first few high-mass X-ray binary (HMXB) pulsars to be discovered. In HMXBs with pulsars such as Vela X−1, the companion’s stellar wind is significantly affected by ionisation due to X-rays from the compact object. An isotopic stellar wind model alone cannot explain the orbital variation in the absorption column density in Vela X−1. A model describing a stream-like photoionisation wake trailing the neutron star has been previously implemented to explain the observed orbital column density variation. Aims. We investigated the variability of the circumbinary environment at different intensity levels of the Vela X−1 and used a model similar to the above-mentioned stream-like photoionisation wake to explain the asymmetric absorption column density present in the source. Methods. The 2.0−20.0 keV MAXI/GSC spectrum was well modelled with a comptonised continuum emission absorbed by local and interstellar material. We used ∼13 years of MAXI/GSC data to constrain the variations in the absorption column density in Vela X−1 obtained from orbital-phase resolved and intensity-and-orbital-phase resolved spectral analysis. Results. The long-term light curve of Vela X−1 shows orbit-to-orbit intensity level variations without any apparent super-orbital periodicity. The orbital-phase resolved spectroscopy in multiple intensity levels reveals asymmetric variation in absorption column density changes across the intensity levels. Conclusions. We confirm that the orbital variation in the absorption column density in Vela X−1 cannot be modelled with a smooth stellar wind alone using ∼13 years of MAXI/GSC data. It requires an additional component, such as a photoionisation wake or an accretion wake. The wake structure is found to be present across different intensity levels of the source, and the geometry of the wake depends on the intensity level. The long-duration MAXI/GSC data allowed us to vary different wake parameters to obtain the best-fit stellar wind parameters for the time-averaged intensity. These best-fit parameters closely reproduce the observed orbital variations in the absorption column density for different intensity levels of the source.