ChandraObservations 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 arederived from a 25 ksec observation by the Chandra X-Ray Observatory. Profilesof the halo, compiled in two energy ranges, 2 to 4 keV and 4 to 6 keV, andthree time intervals before and after an eclipse immersion, exhibit athree-peak shape. The observed profiles are fitted by the profiles of asimulated halo generated by a Monte Carlo ray-tracing code operating on a modelof three discrete clouds and a spectrum of the photons emitted by the sourceover a period of time extending from 270 ksec before the observation began tillit ended. The distances of the two nearer dust clouds are fixed at thedistances of the peaks of atomic hydrogen derived from the 21-cm spectrum inthe direction of the X-ray source, namely at 1.30 and 2.56 kpc. A good fit isachieved with the source at a distance 4.5 kpc, the distance of the third cloudat 4.05 kpc, the total scattering optical depth of the three clouds equal to0.159 at 3 keV, and the column density of hydrogen set to 4.6x10^22 cm^-2. WithAv=6.5 mag for the binary companion star, QV Nor, the ratio of the scatteringoptical depth at 3 keV to the visual extinction is 0.0234 mag^-1.Comment: 36 pages, 15 figures, 4 tables, abridged version of paper accepted for publication in August 1, 2004 Ap