Orbital phase spectroscopy of X - ray pulsars to study the stellar wind of the companion

High Mass X-ray Binary Pulsars (HMXBP), in which the companion star is a source of supersonic stellar wind, provide a laboratory to probe the velocity and density profile of such winds. Here, we have measured the variation of the absorption column density along with other spectral parameters over the binary orbit for two HMXBP in elliptical orbits, as observed with the Rossi X-ray Timing Explorer (RXTE) and the BeppoSAX satellites. A spherically symmetric wind profile was used as a model to compare the observed column density variations. In 4U 1538-52, we find the model corroborating the observations; whereas in GX 301-2, the stellar wind appears to be very clumpy and a smooth symmetric wind model seems to be inadequate in explaining the variation in column density. 4U 1538-52 (Giacconi et al. 1974) and GX 301-2 (White et al. 1976) are both luminous (∼10 35-10 37 ergs s −1) pulsars in elliptical orbits. They emit X-rays due to the accreted matter from the stellar wind of their respective companion stars (Parkes et al. 1978, Kaper et al. 1995). Since both have well defined orbital measurements (Clark 2000, Koh et al. 1997), it enables us to inspect the variation of the X-ray spectral parameters at different orbital phases to gain useful insight into the morphology of the stellar wind. In this work, we have measured the X-ray spectral evolution of these two HMXBP as observed with the RXTE and BeppoSAX satellites and the parameters of the spectral model were examined along the orbit for any possible variations. We have also compared the equivalent hydrogen column density (N H) variations over the