Unexpected variability in the X-ray emission of the isolated neutron star RX J0720.4-3125

During the last three and a half years the isolated neutron star RX J0720.4-3125 has been observed regularly with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. In that period the spectrum of RX J0720.4-3125 changes significantly: In the first observations it can be reasonably well fitted by an absorbed blackbody, but the same model does no longer fit the spectra of the last observations. The flux in the 10 - 23 Å range increases with time, and the flux in the 23 - 38 Å decreases, such that the total flux in the RGS band remains more or less constant during the whole period. Simultaneously with the hardening of the spectrum, the pulse profile of RX J0720.4-3125 becomes narrower, while the hardness-ratio light curve in the first observations leads and in the last observations lags the full-band light curve. If the X-ray emission is affected by the magnetic field of the pulsar, these results would be consistent with precession of the neutron star. We also find possible evidence of variable OVII and OVIII absorption in the RGS spectrum that suggests the presence of a warm absorber in the vicinity of the neutron star, possibly ionized by the energetic pulsar wind.