Column Densities Towards Three Bursting Low-Mass X-ray Binaries from   High Resolution X-ray Spectroscopy
  We measured the galactic hydrogen column densities to the neutron-star binaries GX 17+2, 4U 1705-44, and 4U 1728-34 by modeling the Mg and Si absorption edges found in high-resolution X-ray spectra obtained by the Chandra X-ray Observatory. We found for GX 17+2, N_H = (2.38 +/- 0.12) x 10^22 cm^-2, for 4U 1705-44, N_H = (2.44 +/- 0.09) x 10^22 cm^-2, and for 4U 1728-34, N_H = (2.49 +/- 0.14) x 10^22 cm^-2. These values are in reasonable agreement with the hydrogen column densities inferred earlier from modeling of the continuum spectra of the sources. Our results can be used to constrain the uncertainties of model parameters of the X-ray spectra of these sources that are correlated to the uncertainties of the hydrogen column density. In the case of continuum spectra obtained during thermonuclear X-ray bursts, they will significantly reduce the uncertainties in the spectroscopically measured masses and radii of the neutron stars. 
Patricia Wroblewski, Tolga Guver, and Feryal Ozel (University of   Arizona)