2024MNRAS.533..287L

Hot, $\log (T/\mathrm{K})$$\sim$ 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study, we investigate absorption features of S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ in the spectra of three XRBs, namely 4U 1735-44, 4U 1820-30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2$\sigma$ upper limits for S$_{\mathrm{XVI}}$ K$\alpha$ and Si$_{\mathrm{XIV}}$ K$\alpha$ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at $\log (T/\mathrm{K})$$\gt\ 7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended Circumgalactic Medium.