WE PROPOSE OBSERVATIONS WITH XMM-NEWTON AND NUSTAR OF THE BRIGHT HARD AND PERSISTENT LMXB Z-SOURCE GX13+1 TO (1) MEASURE THE HARD X-RAY HALO BETWEEN 2-8 KEV AND (2) TO MEASURE ITS SPECTRUM AND TIMING CHARACTERISTICS IN THE 5-50 KEV BAND. WITH A BROAD ENERGY RANGE AND MULTIPLE OBSERVATORIES WE WILL REMOVE STATISTICAL AND THUS SYSTEMATIC UNCERTAINTIES IN THE HALO ANALYSIS REVEALING THE SIZE DISTRIBUTION OF LARGE GRAINS ALONG THE LINE OF SIGHT.