Soft X-ray scattering and halos from dust

Small-angle scatterings of X-rays by interstellar dust particles create halos around X-ray sources. The halo intensity and its projected radial distribution around a source can provide important information on the spatial distribution of the dust along the line of sight to the source and on the physical 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 refractory particles with vacuum occupying a significant fraction of their volume. The nature and morphology of interstellar dust particles has recently gained new importance, since huffy, composite dust particles have been suggested as a possible solution to the interstellar carbon "crisis." This crisis results from the discrepancy between the abundance of carbon in the interstellar medium available for creating dust and the significantly larger amount of carbon that must be in dust in order to account for the UV-optical interstellar extinction in the diffuse ISM. Previous studies of X-ray scattering have used the Rayleigh-Gans (RG) approximation to the differential scattering cross section to calculate halo properties. However, the validity of the RG approximation fails for energies below 1 keV. We use the exact Mie solution for the differential scattering cross section and find that, for these energies, the scattering becomes much less efficient than is predicted by the RG approximation. Furthermore, the effects of K and L shell absorption by atoms in the dust become important. The net effect is that the RG approximation systematically and substantially overestimates the intensity of the halo below 1 keV, relative to the Mie solution result. In particular, Mathis and coworkers used the weaker than expected halo intensity observed around Nova Cygni 1992 to conclude that interstellar dust must be fluffy. Using the Mie solution to the scattering intensity and including the effects of absorption, we find that, contrary to the conclusion of Mathis and coworkers, the halo around Nova Cygni 1992 does not require interstellar dust grains to be fluffy in nature and that the data are consistent with scattering from a mixture of bare refractory silicate and carbon grains as well.