2004A&A...419.1077H

The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 show deviations from a Planckian energy distribution below 400 eV, similar to the spectra of RBS1223, another long-period X-ray pulsar, as reported recently by Haberl et al. (2003A&A...403L..19H). For a Gaussian-shaped absorption line we derive an energy of 271eV and an equivalent width of -40eV from the phase-averaged spectra of RX J0720.4-3125. We investigate the spectral variations seen in hardness ratios as function of pulse phase and find that they are best described by changes in the depth of the absorption line. The line equivalent width changes between -31eV around intensity maximum of the pulse and -58eV at the declining part of the pulse. Small variations (<20eV) of the line energy with pulse phase may still be caused by statistical fluctuations. On the other hand, the black-body temperature varies significantly by 2.5eV (statistical 90% errors typically 0.7eV) reaching the highest value at pulse maximum. One possible interpretation for the absorption line is cyclotron resonance scattering of protons in a magnetic field with B ≃5x10¹³G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar.