Confidence contours (68%, 90%, and 99%) in the log T_X vs. log N_H(ISM) plane for the ISM model (cold absorber). The open and filled circles mark the best‐fit parameters (see Table 4). The dashed lines and open symbols represent the confidence contours and best fits for the low S/N observation (1991 April 21), whereas the solid lines and filled symbols represent the confidence contours and best fits for the high S/N observation (1993 April 29). The solid vertical lines mark the respective observed log N_ISM of the four selected stars.