Soft X-ray scattering and halos from dust

Small-angle scatterings of X-rays by interstellar dust particles create hal os around X-ray sources. The halo intensity and its projected radial distri bution around a source can provide important information on the spatial dis tribution of the dust along the line of sight to the source and on the phys ical properties of the scattering dust particles. Halos around X-ray point sources have been used by several authors to infer that the scattering dust particles are fluffy in nature, consisting of aggregates of smaller refrac tory particles with vacuum occupying a significant fraction of their volume . The nature and morphology of interstellar dust particles has recently gai ned new importance, since huffy, composite dust particles have been suggest ed as a possible solution to the interstellar carbon "crisis." This crisis results from the discrepancy between the abundance of carbon in the interst ellar medium available for creating dust and the significantly larger amoun t of carbon that must be in dust in order to account for the UV-optical int erstellar extinction in the diffuse ISM. Previous studies of X-ray scatteri ng have used the Rayleigh-Gans (RG) approximation to the differential scatt ering cross section to calculate halo properties. However, the validity of the RG approximation fails for energies below 1 keV. We use the exact Mie s olution for the differential scattering cross section and find that, for th ese energies, the scattering becomes much less efficient than is predicted by the RG approximation. Furthermore, the effects of K and L shell absorpti on by atoms in the dust become important. The net effect is that the RG app roximation systematically and substantially overestimates the intensity of the halo below 1 keV, relative to the Mie solution result. In particular, M athis and coworkers used the weaker than expected halo intensity observed a round Nova Cygni 1992 to conclude that interstellar dust must be fluffy. Us ing the Mie solution to the scattering intensity and including the effects of absorption, we find that, contrary to the conclusion of Mathis and cowor kers, the halo around Nova Cygni 1992 does not require interstellar dust gr ains to be fluffy in nature and that the data are consistent with scatterin g from a mixture of bare refractory silicate and carbon grains as well.