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The interstellar medium of our Galaxy is a complex medium. While all phases of the ISM can be seen in X-ray absorption towards bright X-ray sources (see previous section), only the hot phase is seen in emission and contributes to the cosmic X-ray background.

This cosmic X-ray background has different components. Kuntz & Snowden (2001) distinguish four different components as outlined below. First, there is an absorbed power-law like component, consisting mostly of unresolved extragalactic point sources. With high spatial resolution like for example available on Chandra, a large part of this component can be resolved into the individual point sources. The second component is the so-called Local Hot Bubble, most likely a large, old supernova remnant embedding our Solar system. The million degree gas of this diffuse medium is almost unabsorbed, as it is nearby and there is little intervening neutral gas. Finally, there are the soft and hard diffuse components, consisting of absorbed, diffuse thermal emission which arises from the disk of our Galaxy but may also contain contributions from a Galactic halo and distant WHIM emission.

Fig. 14 shows a simulated spectrum of the total X-ray background. Note however that the details of this spectrum may differ significantly for different parts of the sky. There are variations on large scales, from the Galactic plane to the poles. There are also smaller scale variations. For instance, there are large loops corresponding to old supernova remnants or (super)bubbles, and large molecular cloud complexes obscuring the more distant background components.

Figure 14

Figure 14. Typical Cosmic X-ray background spectrum. The thick solid line indicates the total spectrum, the dashed line the absorbed power law contribution due to unresolved point sources. The contribution of the local hot bubble is indicated by the thin, grey line. It dominates the line emission below ~ 0.2 keV. The other thermal components are not indicated separately, but they are included in the total; they contribute most of the line flux above 0.2 keV. The calculation is done with a spectral resolution of 1 eV.

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