Properties of the grain-scattered X-ray halo of the eclipsing X-ray binary pulsar 4U 1538-52 are derived from a 25 ksec observation by the Chandra X-ray observatory extending from just before its eclipse immersion to near mid-eclipse .
Profiles of the observed halo , compiled in two energy ranges , 2 to 4 keV and 4 to 6 keV , and three time intervals before and after the eclipse , exhibit a three-peak shape indicative of a concentration of the interstellar dust grains in three discrete clouds along the line of sight .
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 .
In the model , the spectrum before the observation began is expressed as a function of the orbital phase of the pulsar and is derived as an average over 6.3 years of data accumulated by the All Sky Monitor of the Rossi X-Ray Timing Explorer .
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 .
With these constraints , 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 4.6 \times 10 ^ { 22 } cm ^ { -2 } .
With A _ { V } = 6.5 \pm 0.3 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 \pm 0.0010 mag ^ { -1 } .
The column density of hydrogen in the model is much greater than the column density of atomic hydrogen derived from the 21-cm spectrum , which indicates that most of the hydrogen is in molecules , probably concentrated in the three dust clouds .