Orbital Phase Spectroscopy of four High Mass X-ray Binary Pulsars to Study the Stellar Wind of the Companion

TL;DR

This study analyzes the orbital phase-dependent X-ray spectra of four high-mass X-ray binary pulsars to understand the stellar wind properties of their companions, revealing mostly clumpy winds except in one case.

Contribution

It provides the first detailed orbital phase spectroscopy of these pulsars and tests a spherical wind model against observations, highlighting wind clumpiness.

Findings

4U 1538-52's wind fits a spherical model

Other pulsars show evidence of clumpy winds

No disk or gas stream detected in GX 301-2

Abstract

Our work focuses on a comprehensive orbital phase dependent spectroscopy of the four High Mass X-ray Binary Pulsars (HMXBPs) 4U 1538-52, GX 301-2, OAO 1657-415 & Vela X-1. We hereby report the measurements of the variation of the absorption column density and iron-line flux along with other spectral parameters over the binary orbit for the above-mentioned HMXBPs 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. Out of the four pulsars, only in 4U 1538-52, we find the model having a reasonable corroboration with the observations, whereas in the remaining three the stellar wind seems to be clumpy and a smooth symmetric stellar wind model appears to be quite inadequate in explaining the data. Moreover, in GX 301-2, neither the presence of a disk nor a gas stream from the companion was validated. Furthermore, the spectral results obtained in the case of OAO 1657-415 & Vela X-1 were more or less similar to that of GX 301-2.