The properties of the global hot gas on scales comparable to the size of the Galaxy remained largely unknown until recently. Before Chandra, we did have various broad-band X-ray emission surveys of the sky such as the one made by ROSAT in the 0.1-2.4 keV range, which is sensitive to the hot gas . But such a survey alone cannot be used to directly determine the origin of the X-ray emission, which carries little distance information. The interpretation of the emission depends sensitively on the assumed cool (X-ray-absorbing) and hot gas distributions. Spectroscopic information on the X-ray emission has been obtained from rocket experiments  and more recently from Suzaku observations, but only for a number of sample regions (e.g., ). There are also large uncertainties in the contributions from faint discrete sources and other irrelevant phenomena such as solar wind charge exchange (SWCX) to the emission.
Figure 1. Relative line strength (ionization fraction times oscillation strength) of the K lines of key species that trace gas in the temperature range of 105.5 - 106.5 K .
A breakthrough for the study of the global hot gas has been made from the use of the X-ray absorption line spectroscopy (e.g., [12, 13, 14, 15, 16, 17, 18, 19, 20]). While absorption line spectroscopy is commonly used in optical and UV studies of the interstellar medium (ISM), this technique became feasible in the soft X-ray regime only with grating spectra from Chandra and XMM-Newton. Unlike the emission, which is sensitive to the density structure, absorption lines produced by ions such as O VII, O VIII, and Ne IX (Fig. 1) directly probe their column densities, which are proportional to the mass of the hot gas. The relative strengths of such absorption lines give direct diagnostics of the thermal, chemical and/or kinetic properties of the hot gas. Although the absorption lines are rarely resolved in the spectra (with a resolution of ~ 400-500 km s-1 FWHM), the velocity dispersion of the hot gas can be derived from the relative line saturation of different transitions of same ion species (e.g., O VII K vs. K). Because the K-shell transitions of carbon through iron are all in the X-ray regime, the same technique can be used to study the ISM in essentially all phases (cold, warm, and hot) and forms (atomic, molecular, and dust grain; ). Furthermore, the measurements are insensitive to the photo-electric absorption by the cool ISM (kT 104 K) and to the SWCX. Therefore, the X-ray absorption line spectroscopy allows us to probe the global ISM unbiasedly along a sight line. The effectiveness of the technique can be further enhanced when multiple sight lines are analyzed jointly (e.g., ) and/or emission data are included (e.g., ). We can then infer differential properties of hot gas between sight lines of different depths or directions and/or estimate the pathlength and density of the hot gas. The application of this X-ray tomography, though only to a very limited number of sight lines so far, has led to the first characterization of the global hot gas:
This basic characterization demonstrates the potential of using the X-ray tomography to greatly advance our understanding of the global hot gas in and around the Galaxy.