  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. (2003). 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 about 5x10^13 G. This field strength is
compatible with estimates inferred from recent spin down measurements of the
pulsar.
