The scattering of X-rays by interstellar dust on the microarcsecond scale

We have calculated on a small angular scale the distribution of X-rays (e.g., as originating from a point source) scattered by interstellar dust. In addition to the well-known, large-diameter (i.e., hundreds of arcseconds) X-ray halo due to dust distributed along the line of sight to the source, we show that material clumped very close to the source gives rise to a bright, small-diameter (subarcsecond) X-ray halo. The diameter of this inner ``microhalo{''} decreases, and the relative surface brightness increases, as the relative concentration of dust very near the source increases. For example, we expect that the 1 keV X-ray microhalo surrounding a point source in M87 would have a diameter of order 1 microarcsecond (mu as) and a surface brightness relative to the point source flux of order 10(-29) /mu as(-2) cm(2) (H atom)(-1), while the large-diameter halo due to scattering from dust within our own Galaxy would have a diameter of order 100 `` and a relative surface brightness more than 12 orders of magnitude lower than the microhalo surface brightness. We discuss the consequences of our results with regard to future high-resolution X-ray imaging observations, and also with regard to the possibility of using detailed measurements of the X-ray halo profile to determine the spatial distribution of dust along the line of sight and close to the source.