Where is the Supervirial Gas? II. Insight from the Survey of Galactic Sightlines

Abstract Recent observations have revealed a supervirial temperature gas phase at log( T /K) ∼ 7 in the Milky Way, challenging existing galaxy formation models. This hot gas phase was discovered toward extragalactic absorption sightlines and blank-sky emission fields, both at high Galactic latitudes. The location of this hot component is unknown; is it in the extended circumgalactic medium (CGM) or in the interstellar medium (ISM) instead? We analyzed X-ray spectra from Chandra’s High-Energy Transmission Grating observations of 27 Galactic X-ray binaries (XRBs) to investigate whether the hot gas component is present in the ISM. We searched for absorption lines of S xvi K α , Si xiv K α , and Ne x K α , which are the telltale signatures of the hot gas and which have been detected toward extragalactic sightlines. Of the 27 targets, these lines were detected in the spectra of only 7, with two sources displaying broad line features likely intrinsic to the XRB systems. Additionally, most of the detected lines are time variable, reinforcing their likely association with the XRBs. Our results suggest that the supervirial temperature gas is not a widespread component of the ISM but may instead be located in extraplanar regions or the extended CGM, which aligns with some recent simulation results.