  We report the results of a long-term spectral and timing study of the first
transient magnetar, XTE J1810-197 which was discovered in 2003 when its X-ray
luminosity increased 100 fold. We fit X-ray spectra of all archival X-ray
observations using a two-component blackbody model, where the cool component is
most likely originating from the whole surface of the neutron star and the hot
component is from a much smaller hot spot. We investigate the long-term
evolution of the surface emission characteristics via tracing its surface
temperature, apparent emitting area and the pulsed fraction. We evaluate the
pulsed fraction in two energy intervals (<1.5 keV and >1.5 keV) and show that
the XTE J1810-197 exhibits slightly higher pulsed emission at energies above
1.5 keV. We explore the characteristics of an absorption line detected around
1.1 keV. We find that the absorption feature is highly variable and its profile
is asymmetric. To accurately represent this feature, we introduced an
asymmetric Gaussian profile and quantified the level of asymmetry of the
absorption feature.
