Phase-resolved study of the fundamental absorption line for the BeppoSAX observations. Blue points correspond to observation (a), green circles to observation (d), and red diamonds to observation (f). Upper left: pulse profiles extracted in the energy range 11.2–13.2 keV (shifted in phase to be aligned). Upper right: centroid energy of the first cyclotron harmonic. The horizontal cyan band is the energy range of the most negative phase shift of the main peak measured from the SAX data. Lower left: amplitude of the line. Lower right: optical depth of the line.