Long-term variability in the X-ray emission of RX J0720.4-3125
  We detect a gradual, long-term change in the shape of the X-ray spectrum of the isolated neutron star RX J0720.4-3125, such that the spectrum of the source can no longer be described as a blackbody spectrum. The change is accompanied by an energy-dependent change in the pulse profile. If the X-ray emission is influenced by the magnetic field of the pulsar, these changes in spectral shape may point to precession of the neutron star. 
Cor P. de Vries, Jacco Vink, Mariano Mendez, Frank Verbunt