Where is the Supervirial hot gas? I: A pilot study with sightlines to Galactic X-ray binaries

TL;DR

This study investigates the presence of supervirial hot gas in the Milky Way by analyzing X-ray absorption lines toward Galactic X-ray binaries, finding no such lines and suggesting the hot gas detected in extragalactic sightlines is likely outside the ISM.

Contribution

First direct search for supervirial hot gas absorption in the Milky Way's ISM using X-ray binaries, providing upper limits that challenge previous extragalactic detections.

Findings

No detection of ext{SXVI}, ext{SiXIV}, ext{NeX} lines in Galactic sightlines.

Upper limits are an order of magnitude lower than extragalactic detections.

Hot gas at ext{log}T > 7 is unlikely to be ubiquitous in the Galactic ISM.

Abstract

Hot, \logT\ $\sim$ 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 \SXVI, \SiXIV, and \NeX\ 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 \SXVI\ \Ka\ and \SiXIV\ \Ka\ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at \logT\ $>7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that \SXVI, \SiXIV\ {and \NeX\ }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 CGM.