ECLIPSE DOES NOT HIDE, BUT REVEALS: Comprehensive X-ray Reprocessing Studies in High and Low Mass X-ray Binaries with XMM-Newton
Creators: name: Aftab, Nafisa
Publisher: name: Zenodo
Year: 2024
Subjects: X-ray binaries
Fast X-ray transients

description: X-ray reprocessing serves as a vital diagnostic tool for gaining insights into the environments of X-ray binary systems. However, the study of X-ray reprocessing encounters challenges arising from the blending of intense primary radiation from the compact star with the reprocessed radiation from the surrounding. Eclipsing X-ray binaries offer a unique opportunity to investigate pure reprocessed X-rays, as the companion star effectively shields the intense primary X-rays. We carried out comprehensive studies of X-ray reprocessing in several eclipsing High Mass X-ray Binary (HMXB) and Low Mass X-ray Binary (LMXB) systems by comparing their X-ray spectra during and outside of eclipse using XMM-Newton. We found ample diversity in the X-ray reprocessing characteristics in HMXBs, which implies significantly dynamic wind structure surrounding the compact objects in HMXBs. Even in the same source at different epochs, the variation is quite large. Significant differences observed in X-ray reprocessing characteristics in LMXBs despite all being dipping and eclipsing sources, suggest large dependencies of X-ray reprocessing on the inclination angle, scale height of the accretion disk, relative size of the accretion disk with respect to the companion, binary separation, mass ratio between the neutron star and the companion, mass accretion rate, composition of the scattering agents, their density, ionization level, distribution etc. Overall the studies deepen our understanding of the intricate interplay between X-ray reprocessing and the diverse mechanisms within X-ray binary systems. However, in these comprehensive studies with HMXBs and LMXBs we have observed some unexpected behaviours of the X-ray binaries and the exact reason of which is not yet clear. For example, (i) We observed high equivalent widths of Fe emission lines in both Supergiant HMXBs (SgHMXBs) and Supergiant Fast X-ray Transients (SFXTs) during eclipse, indicating a high Fe abundance in both systems. This contrasts with previous studies, which showed high equivalent widths of Fe emission lines in SgHMXBs and low equivalent widths in SFXTs outside of eclipse, suggesting higher Fe abundance in SgHMXBs and lower in SFXTs. (ii) Unlike other SgHMXBs, Cen X-3 exhibited lower equivalent width of Fe Kα emission line during eclipse compared to outside of eclipse, while the equivalent widths of highly ionized Fe emission lines were observed as expected i.e. higher during eclipse compared to outside of eclipse (iii) The low-energy X-ray emission, usually originating from a region near the compact object and expected to be blocked by the supergiant companion during eclipse in SgHMXBs, was not observed to be obscured in the SgHMXB 4U 1538-522 during the eclipse. (iv) The out-of-eclipse to eclipse flux ratio in LMXBs was observed to be smaller compared to that found in HMXBs, indicating greater reprocessing in LMXBs despite their companion stars having less dense and less extended stellar winds. We hope that our discussion in the BRITEstars 2024 meeting will shed some light on these unexpected behaviours of X-ray binaries and will open up many new directions for future research on these systems.