Interstellar absorption and variable soft X-ray component in Cygnus X-1

The hydrogen column density along the line of sight to Cyg X-1 is 7 x 1021 cm-2 as determined from the extinction of its optical counterpart HD 226 868. This value may be used to interpret soft X-ray measurements, including those previously reported, where it is not possible to determine the column density independently from the intrinsic spectral function. The correction for interstellar absorption is larger than previously thought. Application to an old observation suggests that an intense soft X-ray component was present in Cyg X-1, even though the data at approximately keV suggests that it was probably in a low state. This is consistent with the picture of Cyg X-1 suggested by Price and Thorne, in which transitions in Cyg X-1 are attributed to changes in the high energy cut-off of an intense soft component. (Auth.)