Sketch showing (schematically) the most important processes that give rise to WHIM signatures in the X-ray spectra in the direction of a filament (‘ON-FILAMENT’). Those include local production of X-ray photons (green box), resonant and Compton scattering (small red circles), and photo-electric absorption (small blue circles). The region occupied by the WHIM patch is shown with a pink box. The red circles show the scattering of the CXB photons to the line of sight (‘Scattering 1’) and from the line of sight (‘Scattering 2’), respectively. Outside the filament (‘OFF’ spectrum), the observer sees the CXB directly. This simplified picture assumes that all CXB sources are further away than the filament and ignores the foreground completely. Several other processes, e.g. secondary ionization by fast electrons that are produced by photo-ionization, are neglected.