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 ~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.