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Astron. Astrophys. 344, 333-341 (1999)

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1. Introduction

Recent interest in the properties of the Galactic halo arises from two important considerations. Firstly, the observations of galactic gas motions, energetic radio emission from other spiral galaxies and cosmic ray-interstellar gas interactions all appear to support a dynamic halo model, generically known as the fountain model (Shapiro & Field 1976, Kahn 1994). It has been suggested that cosmic rays may drive the vertically upward flow far out (Breitschwerdt et al. 1993). However, according to Bloemen et al. (1993), electron radiation from an edge-on spiral galaxy suggests that energetic electron acceleration and energy loss occurs close to the local disk regions, and not in the thick extended halo. The implication is that structures seen down to the resolution of 350 pc may extend far above the disk. Hence a patchy Galactic halo is plausible.

Secondly, the effect of any halo fluctuations on the diffuse soft X-ray background (SXRB) correlation function may be significant, with the additional possibility of it having an impact on the microwave anisotropy. Hot intracluster gas is an important baryonic component and may contribute up to 30% of the total gravitational mass of rich clusters (White et al. 1993). Suto et al. (1996) have suggested that a halo to our Local Group (LG) could contribute significantly to 1 keV background and also produce a measurable component of the microwave background quadrupole anisotropy. However, Pildis & McGaugh (1996) find data from other poor groups which do not support this hypothesis.

The importance of low energy X-ray data in determining background mass has been highlighted by the detection of excess EUVE emission from the Coma cluster direction (Lieu et al. 1996). The authors suggest that [FORMULA]10% of the gravitational mass is in the form of rapidly cooled [FORMULA]8 . 105 K gas. This observation is dependent on correctly establishing the Galactic background and absorption effects.

In this paper we report on detailed spectral modelling which has been undertaken to better establish the mean and fluctuating components of the Local Hot Bubble (LHB), embedded disk and halo contributions to the SXRB. The data employed are from the ROSAT PSPC observations taken along lines of fixed galactic longitude. A deep observations in the Lockman hole region is also considered. The implication of our results on the galactic structure and on the cosmology-related problems is discussed.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999