The X-ray Halo of GX 13+1
  We present observations of the X-ray halo around the LMXB GX 13+1 from the Chandra X-ray telescope. The halo is caused by scattering in interstellar dust grains, and we use it to diagnose the line-of-sight position, size distribution, and density of the grains. Using the intrinsic energy resolution of Chandra's ACIS CCDs and the recent calibration observation of the Chandra point spread function (PSF), we were able to extract the halo fraction as a function of energy and off-axis angle. We define a new quantity, the ``halo coefficient,'' or the total halo intensity relative to the source at 1 keV, and measure it to be $1.5^{+0.5}_{-0.1}$ for GX 13+1. We find a relationship between this value and the dust size, density, and hydrogen column density along the line of sight to GX 13+1. We also conclude that our data does not agree with ``fluffy'' dust models that earlier X-ray halo observations have supported, and that models including an additional large dust grain population are not supported by these data. 
Randall K. Smith, Richard J. Edgar and Richard A. Shafer