Extreme-ultraviolet and low-energy X-ray scattering from interstellar dust

Although only a few measurements of discrete astronomical objects at X-ray wavelengths have shown the effects of scattering by interstellar dust, there are enough observations of the resulting X-ray haloes to be confident that the scattering process takes place in accordance with the early theoretical predictions by Overbeck and others. All the published data from such X-ray observations are broadly consistent with dust properties measured in other wavelength regions, but the observations made in the energy range from 0.5 to 4.0 keV are not strongly dependent on the detailed parameters of the dust. At lower energies, however, between 20 and 500 eV, the scattering by the kind of dust expected to be present in the interstellar medium is likely to be more critically dependent on the size and composition of the dust particles. Calculations of the scattering and extinction properties of the dust in the EUV-low-energy X-ray band have therefore been made, together with simulations of the radial intensity distribution for scattering haloes produced by different dust distributions. It is apparent from the results presented here that observations at energies below 500 eV, if they can be made successfully, will provide a useful diagnostic of interstellar dust.