X-Ray Reprocessing: Through the Eclipse Spectra of High-mass X-Ray Binaries with XMM-Newton

Abstract

The study of X-ray reprocessing is one of the key diagnostic tools to probe the environment in X-ray binary systems. One difficult aspect of studying X-ray reprocessing is the presence of much brighter primary radiation from the compact star together with the reprocessed radiation. In contrast, for eclipsing systems, the X-rays we receive during eclipse are only those produced by the reprocessing of the emission from the compact star by the surrounding medium. We report results from a spectral study of the X-ray emission during eclipse and outside eclipse in nine high-mass X-ray binaries (HMXBs) with the XMM-Newton European Photon Imaging Camera (EPIC) pn to investigate different aspects of the stellar wind in these HMXBs. During eclipse the continuum component of the spectrum is reduced by a factor of ∼8–237, but the count rate for the 6.4 keV iron emission line or the complex of iron emission lines in HMXBs is reduced by a smaller factor, leading to large equivalent widths of the iron emission lines. This indicates a large size for the line emission region, comparable to or larger than the companion star in these HMXB systems. However, there are significant system to system differences. 4U 1538−522, despite having a large absorption column density, shows a soft emission component with comparable flux during the eclipse and out-of-eclipse phases. Emission from hydrogen-like iron has been observed in LMC X-4 for the first time, in the out-of-eclipse phase in one of the observations.. Overall, we find significant differences in the eclipse spectrum of different HMXBs and also in their eclipse spectra against out-of-eclipse spectra.