Extinction and Distance to Anomalous X-ray Pulsars from X-ray Scattering
  Halos
  We analyze the X-ray scattering halos around three Galactic Anomalous X-ray
Pulsars (AXPs) in order to constrain the distance and the optical extinction of
each source. We obtain surface brightness distributions from EPIC-pn data
obtained with XMM-Newton, compare the profiles of different sources, and fit
them with a model based on the standard theory of X-ray scattering by dust
grains, both for a uniform distribution of dust along the line of sight, and
for dust distributions constrained by previous measurements. Somewhat
surprisingly, we find that for all three sources, the uniform distribution
reproduces the observed surface brightness as well as or better than the
distributions that are informed by previous constraints. Nevertheless, the
inferred total dust columns are robust, and serve to confirm that previous
measurements based on interstellar edges in high-resolution X-ray spectra and
on modelling of broad-band X-ray spectra were reliable. Specifically, we find
Av ~= 4, 6, and 8 mag for 4U 0142+61, 1E 1048.1-5937, and 1RXS
J170849.0-400910, respectively. For 1E 1048.1-5937, this is well in excess of
the extinction expected towards a HI bubble along the line of sight, thus
casting further doubt on the suggested association with the source.
