X-ray reprocessing: Through the eclipse spectra of low mass X-ray binaries with XMM-Newton

TL;DR

This study analyzes X-ray reprocessing in eclipsing low mass X-ray binaries using XMM-Newton data, revealing system-specific features, disk geometries, and reprocessing efficiencies during eclipse and out-of-eclipse phases.

Contribution

It provides the first eclipse spectra for some LMXBs and derives the fractional visible area of accretion disks, highlighting factors influencing reprocessing efficiency.

Findings

Reprocessing characteristics are consistent across intensity states in AX J1745.6$-$2901.

Variable warp structures are indicated in EXO 0748$-$676.

Lower out-of-eclipse to eclipse flux ratios in LMXBs suggest greater reprocessing despite less dense stellar winds.

Abstract

Eclipsing X-ray binaries make an ideal condition to study reprocessed X-rays, as the X-rays detected during eclipse are purely reprocessed while the much brighter primary X-rays are blocked by the companion star. We carried out a comprehensive study of X-ray reprocessing with four eclipsing Low Mass X-ray Binary (LMXB) systems by comparing X-ray spectra during and outside eclipse with \textit{XMM-Newton} EPIC pn observations. The 17 observations of MXB 1659$-$298, AX J1745.6$-$2901, EXO 0748$-$676 and XTE J1710$-$281 give unique features of the systems. For example, X-ray reprocessing characteristics in AX J1745.6$-$2901 is found to be nearly same irrespective of the intensity state; there is an indication of different types of variable warp structures in the inner accretion disk in EXO 0748$-$676, a high out-of-eclipse to eclipse flux ratio in XTE J1710$-$281 inspite of a large size of the accretion disk perhaps indicates low scale height of the accretion disk. The eclipse spectra for some of the LMXB sources are reported for the first time. We have derived the fractional visible area of the accretion disk during maximum eclipse phase for various obscuration geometries. The out-of-eclipse to eclipse flux ratio in LMXBs observed to be smaller compared to that found in High Mass X-ray Binaries. This indicates greater reprocessing in LMXBs despite having less dense, less extended stellar wind from the companion. The X-ray reprocessing efficiencies observed in LMXBs indicate large dependencies of X-ray reprocessing on the scale height of the accretion disk, relative size of the disk compared to the companion and some other unknown factors.