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 ( ) .
For a Gaussian-shaped absorption line we derive an energy of 271 eV and an equivalent width of - 40 eV 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 - 31 eV around intensity maximum of the pulse and - 58 eV at the declining part of the pulse .
Small variations ( < 20 eV ) 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 2.5 eV ( statistical 90 % errors typically 0.7 eV ) 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 \simeq 5 \times 10 ^ { 13 } G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar .