The significance of high-resolution spectral studies in X-ray binaries: current and future

In X-ray binaries, X-rays from the compact object are believed to be reprocessed in the surrounding matter giving rise to fluorescent iron line emission. The neutral Fe line therefore plays a significant role in probing the matter surrounding the compact objects. Changes in neutral Fe fluorescence lines and absorption column density, are often used as tracers of these changing physical conditions in X-ray binaries. These emission lines are ubiquitous to these systems and are powerful probes of the geometry and abundances of surrounding matter near the compact object. Also, at very large values of the ionization parameter, we expect to see He-like and H-like iron emission lines at 6.7 and 6.97 keV, for example. If the Compton optical depth of the medium is high, then the Fe photon at 6.4 keV can be further Compton scattered in the same medium and lose some energy producing a shoulder at a lower energy, known as the Compton Shoulder. We discuss the broad importance of high-resolution studies in X-ray binaries with Chandra gratings combined with broadband studies using NICER and NuSTAR. Taking forward what we have learnt so far, we segue into additional physics that can be investigated in these sources with the microcalorimeter onboard XRISM/Resolve.