Upper limit on dimming of cosmological sources by intergalactic grey dust from the soft X-ray background

Active galactic nuclei (AGN) produce a dominant fraction (F-AGN similar to 80 per cent) of the soft X-ray background (SXB) at photon energies 0.5 < E < 2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray haloes around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F-halo less than or similar to 10 per cent of the SXB can be in the form of diffuse X-ray haloes around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains, with radii in the range a = 0.2-2.0 mu m, to a level tau(GD) less than or similar to 0.15(F-halo/10 per cent)(F-AGN/80 per cent)(-1) to a redshift z similar to 1. Our results are only weakly dependent on the grain size distribution in this size range or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.