The Soft X-Ray Background

Soft X-rays denote the photon energy regime below E = 2 keV. A single sheet of paper is thick enough to block all X-rays of this energy from a source. Soft X-rays from space have been subject of scientific research for more than 30 years. Of particular interest is the ability of the galactic interstellar medium to attenuate the diffuse soft X-ray emission. Quite comparable to medical diagnostics X-rays penetrate thinner parts of the interstellar medium and are more heavily absorbed by denser regions.

ROSAT observations of the diffuse soft X-ray background (SXRB) gave first evidence for the existence of at least three different sources of the SXRB. In addition to the well-known Local Hot Bubble (LHB) ROSAT also discovered the existence of coronal gas located within the Milky Way Halo. Finally, the extragalactic X-ray background contributes to the SXRB emission. In contrast to the two plasmas - both belonging to the Milky Way Galaxy - the extragalactic X-ray background is a superposition of individual X-ray sources located at cosmological distances.

The ROSAT mission discovered "X-ray shadows" of individual Milky Way clouds against the 0.25 keV SXRB for the first time. This discovery led to a revision of the more than 20 years old popular LHB model.

According to the "old" LHB model all SXRB radiation originates within the local void of neutral matter encircling the Sun. The size of the local void is about 100 pc in radial extent. The volume density is extremely low with 0.03 cm^-3 in comparison to the average volume density of the Milky Way interstellar medium with 1 cm^-3. According to the LHB model the local void is filled with coronal gas (T = 10⁶ K) and all intensity variation of the SXRB is proportional to the extent of the line of sight I propto r. The simplicity of the LHB model is also its strength. All X-ray observations prior to the ROSAT mission were consistent with this model. However, some observations, in particular at energies about 0.5 keV, do not match this LHB model.

The revolutionary discovery of ROSAT was the detection of an X-ray shadow of an HI cloud located out-side the local void. This immediately reveals the existence of diffuse coronal gas out-side the LHB and beyond the cloud. The cloud is located within the disk-halo interface region of the Milky Way, the diffuse coronal gas beyond the cloud has to be located within the Galactic Halo. Our group was actively involved in this ROSAT discovery, because the HI gas cloud was one of the favoured observing targets of the Radioastronomisches Institut in HI 21-cm line observations.

It is still a matter of scientific debate what fraction of the SXRB originates within the LHB and what in the Galactic X-ray Halo. Also the physical conditions within both coronal gas phases are actual research topics.

The Bonn group proposed a self-consistent "new" model of the SXRB consisting of three source terms, the LHB, a single phase Milky Way Halo plasma, and the extragalactic X-ray background. According to the "new" model the Milky Way Halo plasma is hotter than the LHB plasma and accounts not only for the 0.25 keV SXRB but also for the 0.75 keV emission. The complete HI of the Milky Way disk is located between the Halo X-ray plasma (vertical scale height 4.4 kpc) and the Sun. This predicts that X-ray shadows are a natural consequence of the SXRB model and not any special features.

Using XMM-Newton we are actually studying the 3-D distribution of the SXRB by analysing the X-ray shadows of individual HI clouds at various distances from the Sun. The huge collecting area of XMM-Newton provides highly significant data. The analysis of ROSAT survey and XMM-Newton data is part of the Ph. D. thesis of Juan Pradas. Some selected publications are available below.

For further information please contact Juan Pradas, Meikel Kappes or Jürgen Kerp.

A Sketch of the Soft X-Ray Background

This figure sketches the origin of the soft X-ray background (SXRB).
Right: a) The assumed constant intensity distribution of I_Halo. b) The distribution of the 0.25 keV absorbing interstellar medium (white = high HI column density, black = low HI column density). c) The SXRB intensity distribution of the constant I_Halo (panel a)) after the transition through the absorbing interstellar medium (panel b)). d) ROSAT all-sky survey data of the same field of interest. The comparison between the model (panel c)) and the observation (panel d)) shows an excellent qualitative agreement between both.
Left: Intensity profiles at b=40 degrees from the maps on the right (see arrows). In panel d) the observed yellow coded intensity profile is superposed by the thick red line, which represents the modelled intensity distribution. The intensity profiles show that not only qualitatively but also quantitatively the SXRB can be modelled on angular scales of tens of degrees.

Selected Publications

• Kappes M., Kerp J., 2003 "A window to the Galactic X-ray halo: The ISM towards the Lockman hole" Astronomical Notes 324, 149 see astro-ph/0211212

• Kappes M., Kerp J., Richter P., 2003 "The composition of the interstellar medium towards the Lockman Hole" Astronomy &Astrophysics 405, 607 see astro-ph/0304505

• Kerp J., 2003 "HI, the Window to the Early Universe in X-rays" Astronomical Notes 324, 69

• Pradas J., Kerp J., 2003 "The 3-D composition of the galactic interstellar medium. The hot phases and the X-ray absorbing material." Astronomical Notes 324, 151 see ADS

• Pradas J., Kerp J., Kalberla, P.M.W. 2003 "The soft X-ray background towards the northern sky. A detailed analysis of the Milky Way Halo." Astronomical Notes 324, 150

Co-Investigators

We like to mention our co-investigators: Dr. Michael Dahlem (CSIRO, Australia), Dr. Matthias Ehle (Vilspa, Spain), Dr. Fred Jansen (ESTEC, The Netherlands).

Acknowledgements

We are grateful for the financial support by the Deutsches Zentrum für Luft- und Raumfahrt under grant No. 50 OR 0103.