Unveiling the structures of Stellar Wind in X-ray Binaries using MAXI/GSC

In X-ray binaries, the companion star’s stellar wind is strongly influenced by the intense gravity and ionizing X-rays from the compact object. This interaction may lead to an accretion wake, driven by the compact object’s gravity and a photoionization wake, due to X-ray ionization of the stellar wind. These systems also exhibit X-ray intensity variations from orbit to orbit. Using MAXI/GSC, we study the orbital phase dependence of the spectrum at different intensity levels of the source. In high-mass X-ray binaries(HMXBs) like Vela X-1, the orbital variation of absorption column density shows that a simple isotropic stellar wind model cannot account for observed anisotropy. A stream-like photoionization wake trailing the neutron star was previously used to explain the asymmetric absorption in Vela X-1. To explore long-term variations, we analyze nearly ~13 years of MAXI/GSCdata, dividing the total intensity into multiple levels. Our results suggest that the accretion/photoionization-like structure persists across intensity levels, with the geometry of the wake varying accordingly. The combination of a smooth stellar wind and a trailing accretion/photoionization wake explains the variations in absorption column density, with wind parameters well-constrained by the data. We use similar techniques to study other X-ray binaries. In low-mass X-ray binaries(LMXBs) like Her X-1, a strong stellar wind is unlikely, with Roche lobe overflow(RL-overflow) being the primary mechanism. Another HMXB, LMC X-4, also undergoes RL-overflow due to its small orbital period. We observed spectral hardening in Her X-1 before the eclipse, likely due to pre-eclipse dips, a feature observed in some LMXBs during some binary orbits which may be due to a puffed-up outer accretion disk.