Chandra observations and Monte Carlo simulations of the grain-scattered halo of the binary X-ray pulsar 4U 1538-52

Properties of the grain-scattered X-ray halo of the eclipsing X-ray binary pulsar 4U 1538-52 are derived from a 25 ks observation by the Chandra X-Ray Observatory extending from just before its eclipse immersion to near mideclipse. Profiles of the observed halo compiled in two energy ranges, 2-4 and 4-6 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 ks before the observation began until 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 yr 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 4.6x10(22) cm(-2). With A(V)=6.5+/-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+/-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.