Post-glitch variability in the anomalous X-ray pulsar 1RXSJ170849.0-400910

Here we report on the first XMM-Newton observation of the anomalous X-ray pulsar 1RXSJ170849.0-400910. The source was observed in 2003 August, and was found at a flux level factor of about 2 lower than previous observations. Moreover, a significant spectral evolution appears to be present, the source exhibiting a much softer spectrum than in the past. Comparison of the present properties of 1RXSJ170849.0-400910 with those from archival data shows a clear correlation between the X-ray flux and the spectral hardness. In particular, the flux and the spectral hardness reached a maximum level close to the two glitches the source experienced in 1999 and 2001, and successively decreased. Although the excellent XMM-Newton spectral resolution should in principle allow us to detect the absorption line reported in a phase-resolved spectrum with BeppoSAX, and interpreted as a cyclotron feature, we found no absorption features, neither in the phase-averaged spectrum nor in the phase-resolved spectra. We discuss in detail both the possibilities that the feature in the BeppoSAX data may have resulted from a spurious detection and that it is real and intrinsically variable. We then discuss a possible explanation for the glitches and for the softening of the source emission that followed the flux decrease, in the framework of the magnetar model.