From outburst to quiescence: the decay of the transient AXP XTE J1810-197

Aims. XTE J1810-197 is the first transient anomalous X-ray pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. Methods. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four year baseline (September 2003 September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse-phase spectroscopic studies were also carried out for observations with a high enough signal-to-noise. Results. We find that (i) a three-blackbody model reproduces the spectral properties of XTE J1810-197 over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005). If this stems from proton resonant cyclotron scattering, it would imply a magnetic field of similar to 2 x 10(14) G. This closely agrees with the value from the spin period measurements.