X-ray scattering by nonspherical grains. I. Oblate spheroids

We calculate the scattering of X-rays by interstellar dust for a dust model that reproduces the observed wavelength-dependent extinction and polarization of starlight. On interstellar sight lines that produce appreciable starlight polarization, we predict that the dust-scattered X-ray halo around point sources will have measurable azimuthal asymmetry due to scattering by partially aligned nonspherical grains. We calculate the expected halo asymmetry. X-ray halo asymmetry provides a new test of interstellar dust models.