2003A&A...405..607K

The Lockman Hole is well known as the region with the lowest neutral atomic hydrogen column density on the entire sky. We present an analysis of the soft X-ray background radiation towards the Lockman Hole using ROSAT all-sky survey data. This data is correlated with the Leiden/Dwingeloo survey (Galactic HI 21cm-line emission) in order to model the soft X-ray background by using radiative transfer calculations for four ROSAT energy bands simultaneously. It turns out, that an important gas fraction, ranging between 20-50%, of the X-ray absorbing material is not entirely traced by the HI but is in the form of ionized hydrogen. Far-ultraviolet absorption line measurements by FUSE are consistent with this finding and support an ionized hydrogen component towards the Lockman Hole.