Aa 376, 248--253 (2001)

XMM-Newton observations of the X-ray feature 1SAX J0617.1+2221 in the IC 443 supernova remnant are reported. We resolve the structure of the nebula into a compact core with a hard spectrum of photon index # = 1.63 in the 2--10 keV energy range. The nebula also has an extended (#8 # 5 # ) X-ray halo, much larger than the radio emission extension. The photon index softens, following a linear scaling with distance from the centroid, similar to other known X-ray plerions. The index range is compatible with synchrotron burn-o# models. All the observational evidence points toward a confirmation of the plerionic nature of the nebula, as recently suggested by a Chandra observation, but with characteristics more similar to "non Crab-like" plerions. We discuss the implications on the synchrotron nebula magnetic field if the >100 MeV emission reported by CGRO EGRET is produced by the synchrotron emission. We also constrain the thermal emission of the central object, arguing that the surface temperature should be around 0.1 keV, although other possible fits cannot be excluded on the base of the XMM-Newton data