Discovery of a Strongly Phase-variable Spectral Feature in the Isolated Neutron Star RX J0720.4-3125

We present the discovery of a strongly phase-variable absorption feature in the X-ray spectrum of the nearby, thermally emitting, isolated neutron star RX J0720.4-3125. The absorption line was detected performing detailed phase-resolved spectroscopy in 20 XMM-Newton observations, covering the period 2000 May-2012 September. The feature has an energy of ~750 eV, an equivalent width of ~30 eV, and it is significantly detected for only ~20% of the pulsar rotation. The absorption feature appears to be stable over the timespan covered by the observations. Given its strong dependence on the pulsar rotational phase and its narrow width, a plausible interpretation is in terms of resonant proton cyclotron absorption/scattering in a confined magnetic structure very close to the neutron star surface. The inferred field in such a magnetic loop is Bloop ∼ 2 x times 10 G, a factor of ~7 higher than the surface dipolar magnetic field.