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 similar to 750 eV, an equivalent width of similar to 30 eV, and it is significantly detected for only similar to 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 B-loop similar to 2 x10 (14) G, a factor of similar to 7 higher than the surface dipolar magnetic field.