A Shell of Thermal X-Ray Emission Surrounding the Young Crab-like Remnant 3C 58

Deep X-ray imaging spectroscopy of the bright pulsar wind nebula 3C 58 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×(Ne/Ne_solar) 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)×10²¹ cm^-2. Our result suggests that thermal shells can be obscured in the early evolution of a supernova remnant by nonthermal pulsar wind nebulae emission; the luminosity of the 3C 58 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 3C 58 is not the remnant of the supernova of CE 1181.