Interstellar matter column density to CEN X-3

The usual gas-to-dust ratio was shown to hold quite precisely for a sample of supernova remnants with available x-ray spectra and interstellar reddening observations. Supernova remnants are extended objects, tenuous enough to be optically thin in the x-ray range; it appears that the x-ray observations can readily be interpreted in terms of a main source component and of an interstellar perturbation affecting the lower part of the spectrum. The value N/sub H//E/sub B-V/ = (6.8 +- 1.5) x 1021 H atoms cm-2 mag -1 was found to hold, without any sizeable systematic deviation up to E/sub B-V/ approximately 1.7. Here, N/sub H/ expresses the total hydrogen column density, including ionized, atomic, and molecular forms. Cen X-3 is associated with an OB supergiant, and E/sub B-V/ = 1.29 mag; the equation thus yields N/sub H/ = 8.8 x 1021 H atoms cm-2. The effect of such a column density is easily detectable in the x-ray range. The spectrum exhibits a low energy cut-off, which, according to the usual practice, is parametrized by a column of cold matter, N/sub X/. The cut-off is definitely observed to be variable, and values of N/sub X/ range from approximately 0 to 1.5 x 1023 H atoms cm-2. There is a suggestion that sometimes N/sub X/ is less than N/sub H/. It may be concluded that on those occasions a spurious soft x-ray component is present in the source, bearing close similarity with Cyg X-1. When N/sub X/ is large, self-absorption in an optically thick gas is very likely to take place