Interstellar Dust seen with X-ray Halos
Randall Smith
X-ray scattering by micron-sized or smaller particles is a powerful diagnostic
of size, shape, and composition. In the laboratory, this technique is used
to measure the shape of small proteins and other biologic materials. In
interstellar space, X-ray scattering leads to diffuse "halos" around highly
absorbed sources. I will discuss the physics of the scattering, both in the
Rayleigh-Gans approximation and the full Mie solution, and show how it
provides a new approach to studying the large interstellar dust grains that
contain most of the mass. I will then present X-ray halo observations done
with the Chandra X-ray telescope around a number of sources, including highly
absorbed supernova remnants and neutron stars. Existing interstellar dust
studies rely largely on UV/optical scattering, which are effective only for
lightly-absorbed sources. As a result, X-ray halos provide a new window for
studying large grains in dense clouds--material about which little is currently
known, although of great interest since it must condense (in at least some
cases) to form planets like the Earth.