Survey of Column Densities to Galactic X-ray Sources Using the Einstein OGS Vrtilek, S. D. ; Wargelin, B. J. ; McClintock, J. E. ; Kahn, S. M. ; Seward, F. D. Abstract Publication: Bulletin of the American Astronomical Society Pub Date: September 1987 Bibcode: 1987BAAS...19.1076V