The composition of the interstellar medium towards the Lockman Hole

, UV and X-ray observations

¹ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: jkerp@astro.uni-bonn.de
² Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Florence, Italy e-mail: richter@arcetri.astro.it

Corresponding author: M. Kappes, mkappes@astro.uni-bonn.de

Received: 21 January 2003
Accepted: 18 April 2003

Abstract

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  21 cm-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 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.