Empirical relation between interstellar x-ray absorption and optical extinction

An empirical relation between interstellar X-ray absorption and optical extinction is derived from the correlation of measurements made on objects of large intrinsic diameter. The result is A/subv/=4.5times10-22 N/subH/ mag, with the principal error being largely systematic in origin, where N/subH/ represents the column density of interstellar matter in the Brown and Gould model for the X-ray absorption coefficient. Applying this ratio to optically identified compact sources, it is concluded that sources in binary systems showing pronounced X-ray occultations have an intrinsic absorption equivalent to approx.1022 atoms cm-2 of interstellar matter and that there are a few compact sources where the absorption seems to be primarily interstellar in origin. The interstellar absorption expected in Cyg X-1 from the extinction of its optical counterpart is much greater than that suggested by X-ray spectra, which may be due to a soft X-ray component greater than that predicted by the power law fitted to higher energy data. A reinterpretation of published spectral parameters of Cyg X-1 in its posttransition phase suggests that the soft component shifted downward in energy during the transition. The value of A/sub v//N/sub H/ determined from the X-ray measurements is compatible with the Lα results for 0A0 2. It is in accord with an interstellar dust-to-mass ratio of approximately 10-2. (U.S.)