ROSAT Galactic and Extragalactic X-ray Shadowing Experiments

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Session 21 -- Diffuse Galactic Emission
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[21.06] ROSAT Galactic and Extragalactic X-ray Shadowing Experiments

Q. Daniel Wang (JILA/CASA/Univ. of Colorado)

To understand the nature of the soft X-ray background and to use the background as constraints on the populations and properties of various X-ray-emitting sources, we are conducting a systematic ROSAT study of X-ray shadows cast by Galactic and extragalactic gas clouds. We present some preliminary results of this study.

A shadow is detected at $\sim$ 1/4 keV in regions covered by H\ I gas associated with the disk of the nearby edge-on galaxy NGC4631 and with tidal tails produced by the interaction between the galaxy and its companions. Shadows at $\sim$ 3/4 keV and 1.5 keV are observed in an experiment with a neutral gas cloud located in the Magellanic Bridge (an H\ I feature connecting the Large and Small Magellanic Clouds). These experiments, free from the confusion of diffuse X-ray emission associated with our Milky Way, allow us to measure the extragalactic background radiation in the 0.2 - 2 keV range and the integrated Galactic X-ray emission along the lines of sight. This, together with the results from our autocorrelation analysis of ROSAT deep observations (Wang \& McCray 1993, ApJL 409, 37), can be used to place constraints on the population and evolution of various X-ray sources.

We have also detected many X-ray shadows produced by Galactic gas clouds (e.g., IR cirrus, molecular clouds). We show examples to illustrate how these shadows can be used to understand both the cold and hot interstellar medium. Since the distances to these clouds are known, or may be determined, the measurements of the foreground and background X-ray emissions relative to these clouds provide us with important information about line-of-sight properties of the hot medium. Furthermore, X-ray shadows can be used to measure the gas column density distribution and mass of molecular clouds, providing important calibrators for molecular cloud astrophysics.

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