Abstract
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.
Highlights
In X-ray binary systems, the X-rays produced very close to the compact object fuelled by accretion are called primary X-rays
During eclipse the continuum component of the spectrum is reduced by a factor of ∼8–237, but the count-rate for 6.4 keV Iron emission line or complex of Iron emission lines in high mass X-ray binaries (HMXBs) are reduced by a smaller factor leading to large equivalent widths of the Iron emission lines
The eclipse spectrum of this observation is best fitted with a powerlaw, two Gaussian functions with energies 6.41 keV and 6.88 keV modified by photoelectric absorption and a less absorbed blackbody emission associated with the line of sight equivalent Hydrogen column density of
Summary
In X-ray binary systems, the X-rays produced very close to the compact object fuelled by accretion are called primary X-rays. Naik & Paul (2012) with the same XMM-Newton observation which we have analysed here, found significant variability in the line parameters of the three Fe Kα emission lines during eclipse and out-of-eclipse phases They suggest that the colder material, causing 6.4 keV line emission, is relatively close to the neutron star at most on the scale of the companion, while the hot matter producing the Fe XXV and Fe XXVI emission lines are further away as suggested by Ebisawa et al (1996). Suzaku observation of Cen X-3 covering a full binary orbit showed multiple extended dips with spectral characteristics similar to that of the source in eclipse, indicating the dips to be produced due to obscuration by dense matter which are structures in the outer region of the accretion disk (Naik, Paul & Ali 2011). We try to infer the X-ray wind characteristics of these systems, which is the main reprocessing agent in the high mass X-ray binaries
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