  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. We analyzed nine XMM-Newton observations of XTE J1810-197
collected over a four years 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
sufficiently high signal to noise. We find that: (i) a three blackbodies model
reproduces the spectral properties of the source 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 due to proton resonant cyclotron scattering, it would imply a
magnetic field of around 2E14 G. This is in a very good agreement with the
value from the spin period measurements.
