The X-ray halo of GX 13+1

We present observations of the X-ray halo around the low-mass X-ray binary 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, we were able to extract the halo fraction as a function of energy and off-axis angle. We de ne a new quantity, the "halo coefficient", or the total halo intensity relative to the source extrapolated to 1 keV, and measure it to be 1.5(-0.1)(+0.5) 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 do 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.