  Deep X-ray imaging spectroscopy of the bright pulsar wind nebula 3C58
confirms the existence of an embedded thermal X-ray shell surrounding the
pulsar PSR J0205+6449. Radially resolved spectra obtained with the XMM-Newton
telescope are well-characterized by a power-law model with the addition of a
soft thermal emission component in varying proportions. These fits reproduce
the well-studied increase in the spectral index with radius attributed to
synchrotron burn-off of high energy electrons. Most interestingly, a radially
resolved thermal component is shown to map out a shell-like structure ~6' in
diameter. The presence of a strong emission line corresponding to the Ne IX
He-like transition requires an overabundance of ~3 x [Ne/Ne(sun)] in the
Raymond-Smith plasma model. The best-fit temperature kT ~ 0.23 keV is
essentially independent of radius for the derived column density of N_H = (4.2
+/- 0.1)E21 per cm squared. Our result suggests that thermal shells can be
obscured in the early evolution of a supernova remnant by non-thermal pulsar
wind nebulae emission; the luminosity of the 3C58 shell is more than an order
of magnitude below the upper limit on a similar shell in the Crab Nebula. We
find the shell centroid to be offset from the pulsar location. If this neutron
star has a velocity similar to that of the Crab pulsar, we derive an age of
3700 yr and a velocity vector aligned with the long axis of the PWN. The shell
parameters and pulsar offset add to the accumulating evidence that 3C58 is not
the remnant of the supernova of CE 1181.
