  We present the results of an XMM-Newton observation of the radio-quiet X-ray
pulsar 1E 1207-5209 located at the center of the shell-like supernova remnant
G296.5+10.0. The X-ray spectrum is characterized by the presence of two
phase-dependent absorption lines at energies ~0.7 keV and ~1.4 keV. Moreover,
these broad spectral features have significant substructure, suggesting that
they are due to the blending of several narrower lines. We interpret such
features as evidence for an atmosphere containing metals and a magnetic field
value of a few 10^12 G, consistent with the observed spin-down rate
P_dot=(1.98+/-0.83)*10^(-14) s/s. Since 1E 1207-5209 is the only X-ray emitting
pulsar showing evidence of such features, we tentatively link them to the
unique combination of age and energetics that characterize this object. We
suggest that a young age and a low level of magnetospheric activity are
favorable conditions for the detection of atomic spectral features from metals
in neutron star atmospheres, which would be either blanketed by a thin layer of
accreted hydrogen in older objects or masked by non-thermal processes in young
energetic pulsars.
