Constraining the Porosities of Interstellar Dust Grains

We present theoretical calculations of the X-ray scattering properties of porous grain aggregates with olivine monomers. The small and large angle scattering properties of these aggregates are governed by the global structure and substructure of the grain, respectively. We construct two diagnostics, R_X and T_X, based on the optical and X-ray properties of the aggregates, and apply them to a Chandra measurement of the dust halo around the Galactic binary GX13+1. Grain aggregates with porosities higher than about 0.55 are ruled out. Future high-precision observations of X-ray dust haloes together with detailed modeling of the X-ray scattering properties of porous grain mixtures will further constrain the presence of porous grain aggregates in a given dust population.