Monte Carlo Simulation of the X-Ray Halos of GX 5-1 and GX 13+1 to Test Models of Insterstellar Dust Grains

Simulated dust-scattered X-ray halos, generated by a three-dimensional ray-tracing Monte Carlo code, are fitted to the observed halos of GX 5-1 and GX 13+1 by automatic adjustment of parameters that specify the distributions of the dust grains in size and location. Three distributions in size a are tested for the quality of fit they yield: the WD01 distribution, the BARE-GR-B distribution, and power laws of the form dn/da similar to a(-q). The best fits are obtained with multicloud models of the spatial distributions and power-law size distributions. Correlations between the fitted fractional distances of the dust clouds and the distance estimates of molecular clouds toward GX 5-1 and GX 13+1, derived from the velocity spectra of CO and the galactic rotation curve, favor distance estimates of 4.2 and 5.6 kpc for the two sources, respectively.