Constituents of the soft X-ray background

Context. The X-ray background (XRB) is generated by various classes of objects and a variety of emission mechanisms. Relative contribution of individual components depends on energy. Aims. The goal is to assess the integral emission of the major components of the soft XRB (extragalactic discrete sources dominated by active galactic nuclei (AGN), galactic plasma, and the Warm/Hot Intergalactic Medium [ WHIM]), investigating the angular structure of the background. Methods. Fluctuations of the background are measured using the auto-correlation function of the XRB determined in 5 energy bands between 0.3 and 4.5 keV. The investigation is based on the extensive observational data set selected from the XMM- Newton archives. Results. Amplitudes of the auto-correlation functions calculated in three energy bands above similar to 1 keV are consistent with the conjecture that the background fluctuations result solely from clustering of sources which produce the background. At energies below 1 keV the relative fluctuation amplitude decreases indicating that a fraction of the soft XRB is associated with a smooth plasma emission in the Galaxy. It is shown, however, that the mean spectrum of extragalactic discrete sources steepens in the soft X-rays and is not well represented by a single power law in the energy range 0.3-4.5 keV. The WHIM contribution to the total background fluctuations is small and consistent with the WHIM properties derived from the cross-correlation of the XRB with galaxies.