X-ray background field spectra. The black solid line shows a cluster X-ray background field as proposed by Adams et al. (2012) with a flux of 2 × 10^-5 erg cm^-2 s^-1 modelled with a bremsstrahlungs spectrum with T_X = 2 × 10⁷K. The red and blue solid lines are for fluxes ten times higher and lower, respectively. For comparison we also show the diffuse extragalactic X-ray background field (XRB, brown solid line) using the fits described in Fabian & Barcons (1992) and a hot-gas spectrum (magenta line) from, for example, Supernova remnants (see Tielens 2005 chap. 1; Slavin & Frisch 2008).