Pulse phase resolved EPIC-pn spectra of RX J0720.4-3125 observed by XMM-Newton: a reproduction of Fig. 5 of Haberl (2007), showing an absorption feature around 300 eV, which is interpreted as a proton cyclotron line produced in a magnetic field of about 5.6 × 10¹³ Gauss. The different colors give the spectra for different pulse phase intervals: 0.0–0.2: black, 0.2–0.4: red, 0.4–0.6: green, 0.6–0.8: blue, and 0.8–1.0: light blue.