THE SOFT-X-RAY BACKGROUND TOWARDS THE HIGH-VELOCITY-CLOUD COMPLEX-M -HI CLOUDS ASSOCIATED WITH THE HOT GALACTIC CORONA

Observations with ROSAT have established that the emission of a diffus e hot plasma in the galactic halo contributes to the X-ray background at 1/4 keV. This paper addresses the question of the extent of this ho t galactic corona and discusses the origin of the soft X-rays emitted there. Based on the Wisconsin soft X-ray surveys, Hirth et al. suggest ed that the interaction of HI high-velocity clouds with the galactic d isk gas is at least partly responsible for the soft X-ray background. Here we compare the ROSAT 1/4 keV survey data towards the high-velocit y-cloud Complex M with the new Leiden/Dwingeloo survey of galactic HI. The distance of this cloud complex with respect to the galactic plane has recently been constrained to lie between 1.5 and 4.4 kpc. To acco unt for the observed distribution of soft X-ray emission, we consider the absorption of the X-ray photons by the neutral foreground gas incl uding intermediate-and high-velocity neutral clouds. We find that the high-velocity clouds attenuate the soft X-ray background. This shows t hat the galactic corona extends at least up to several kiloparsecs fro m the galactic plane. Furthermore, we find evidence for a soft Xray ed ge-brightening of parts of the high-velocity clouds, suggesting that t hese clouds are contributing to the heating of the galactic corona. Th e heating mechanism itself remains unclear.