Chandra Observations and Monte Carlo Simulations of the Grain-Scattered   Halo of the Binary X-Ray Pulsar 4U 1538-52
  Properties of the X-ray halo of the eclipsing X-ray pulsar 4U 1538-52 are derived from a 25 ksec observation by the Chandra X-Ray Observatory. Profiles of the halo, compiled in two energy ranges, 2 to 4 keV and 4 to 6 keV, and three time intervals before and after an eclipse immersion, exhibit a three-peak shape. The observed profiles are fitted by the profiles of a simulated halo generated by a Monte Carlo ray-tracing code operating on a model of three discrete clouds and a spectrum of the photons emitted by the source over a period of time extending from 270 ksec before the observation began till it ended. The distances of the two nearer dust clouds are fixed at the distances of the peaks of atomic hydrogen derived from the 21-cm spectrum in the direction of the X-ray source, namely at 1.30 and 2.56 kpc. A good fit is achieved with the source at a distance 4.5 kpc, the distance of the third cloud at 4.05 kpc, the total scattering optical depth of the three clouds equal to 0.159 at 3 keV, and the column density of hydrogen set to 4.6x10^22 cm^-2. With Av=6.5 mag for the binary companion star, QV Nor, the ratio of the scattering optical depth at 3 keV to the visual extinction is 0.0234 mag^-1. 
George W. Clark