Distance r of the absorber, in units of 10⁹ cm, from the NS with respect to its thickness Δr (in units of 10⁹ cm). The gray area indicates the possible pairs of r and Δr. The horizontal dashed line indicates the best-fit value of r obtained assuming that the observed redshift is due to gravitational redshift for a NS mass of 1.4 M_⊙; the green area takes into account the associated errors. The black, red, and blue lines are the constraint conditions (see text).