Our simple model continues to provide meaningful spectral fits over a large area of the sky, signifying the basic validity of the assumptions about the geometry. Residual instrument calibration uncertainties, particle contamination and scattered solar X-ray background are likely to be the principal sources of error. The discrepancies may also be attributable to long term enhancements which tend to have softer component than the cosmic ray background. Comparison of the best fit halo emission measures found as a function of position lend no positive support to the hypothesis that there is a significant hot gas distribution centred on the LG. A patchy Galactic halo hot gas at temperature log T = 6.3 and with an average EM of 0.02 cm-6 pc is much more likely.
The electron density we would attribute to any LG halo is too small to affect planned investigations of the Cosmic Microwave Background (CMB) anisotropy. It is worth noting that this electron density is only 1/10th of that of the local galactic halo. The observed fluctuations in the 106 K halo render it impossible to investigate the 1 keV SXRB spatial variability to better than 10% accuracy and likewise the 2 keV SXRB to better than 1%.
The spectral fitting in the Lockman hole region in comparison with the all-sky average suggests fluctuations in the power law representation of the intrinsic SXRB by approximately 50%, although we cannot be certain of separating local and true extragalactic sources of spatial variability. We wish to stress the difficulty in the 1-2 keV energy regime of effecting this separation.
Halo EM fluctuations are established at the 30% level over an angular scale of 5o (Table 1). Since there is likely to be less fluctuation power at smaller angular separations, claims that certain clusters of galaxies exhibit EUV excess (e.g. Lieu at al. 1996) are probably safe from this source of contamination as the spatial scales involved are smaller.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999