Spectral and Temporal Variations of the Isolated Neutron Star RX J0720.4-3125: New XMM-Newton Observations

Context: In the past, the isolated, radio-quiet neutron star RX J0720.4-3125 showed variations in its spectral parameters (apparent radius, temperature of the emitting area and equivalent width of the absorption feature) seen in the X-ray spectra, not only during the spin period of 8.39 s, but also over time scales of years. New X-ray observations of RX J0720.4-3125 with XMM-Newton extend the coverage to about 7.5 years with the latest pointing performed in November 2007. Out of a total of fourteen available EPIC-pn datasets, eleven have been obtained with an identical instrumental setup (full frame read-out mode with a thin filter), and are best suited for a comparative investigation of the spectral and timing properties of this enigmatic X-ray pulsar. Aims: We analysed the new XMM-Newton observations together with archival data in order to follow the spectral and temporal evolution of RX J0720.4-3125. Methods: All XMM-Newton data were reduced with the standard XMM-SAS software package. A systematic and consistent data reduction of all these observations is warranted in order to reduce systematic errors as far as possible. Results: We investigate the phase residuals derived from data from different energy bands using different timing solutions for the spin period evolution and confirm the phase lag between hard and soft photons. The phase shift in the X-ray pulses between hard and soft photons varies with time and changes sign around MJD=52 800 days, regardless of the chosen timing solution. The phase residuals show a marked dependence on the energy band, and possibly follow a cyclic pattern with a period of ~9-12 yrs. We find that an abs(sine) dependence provides a better fit to the residuals with respect to a simple sine wave, although in both cases the reduced χ2 is not completely satisfactory. We compared the model fit to phase residuals derived from EPIC-MOS and Chandra (HRC and ACIS) data restricted to the hard energy band (400-1000 eV) to take into account the different energy responses of these instruments. Conclusions: The new data are not in contradiction with RX J0720.4-3125 being a precessing neutron star but do not provide overwhelming evidence for this picture either.