1 Introduction

The processes of conversion of pulsar spin-down energy into high-energy emission in pulsar wind nebula are of great physical interest (e.g., Kennel & Coroniti 1984; Chevalier 2000). Modern arcsec resolution instruments such as on XMM-Newton and Chandra that are sensitive to photons up to 10 keV provide unique possibilities to study pulsar nebulae, because the non-thermal emission of the nebulae may be easily detected and studied in this band (e.g., Vela-X, Helfand et al. 2001; Pavlov et al. 2001; G21.5-0.9, Slane et al. 2000; Warwick et al. 2001; and 3C 58, Bocchino et al. 2001), even when immersed in the soft X-ray emission of the companion shell, as is often the case.

Indeed, the shell SNR IC 443, once though to be mostly thermal in the X-ray band (Petre et al. 1988; Asaoka & Aschenbach 1994) has been discovered to emit hard X-ray emission by Wang et al. (1992). ASCA Gas Imaging Scintillator (GIS) observations by Keohane et al. (1997) discovered the localized character of the hard X-ray emission and its non-thermal nature. They concluded that most of the 2-10 keV photons came from an isolated emitting feature and from the South East elongated ridge of hard emission. Preite-Martinez et al. (1999) and Bocchino & Bykov (2000) reported a hard component detected with the Phoswich Detector System (PDS) on BeppoSAX and two compact X-ray sources corresponding to the ASCA sources detected with the BeppoSAX Medium-Energy Concentrator Spectrometer (MECS) (1SAX J0617.1+2221 and 1SAX J0618.0+2227). Very recently, 1SAX J0617.1+2221 has been observed by Chandra as reported by Olbert et al. (2001), who also show a VLA observation at 1.46, 4.86 and 8.46 GHz, and a polarization measurement. They argue that the hard radio spectral index, the amount of polarization and the overall X-ray and radio morphology strongly suggest that the source is a plerion nebula with a point source in it, whose characteristic cometary shape is due to supersonic motion of the neutron star. However, the limited counting statistics of the 10 ks Chandra observation do not allow a detailed spectral study of the nebula, which is required to compare this new plerion with current theoretical models. Moreover, IC 443 is also a possible candidate for the CGRO EGRET $\gamma$-ray source 3EG J0617+2238 (Hartman et al. 1999) having a flux above 100 MeV of ${\sim} 5 \times 10^{-7}$ ph s^-1 cm^-2 with a photon index of $2.01 \pm 0.06$. More detailed measurements of the nebula spectral properties are needed to study the relation between 3EG J0617+2238 and the nebula.

In this paper an XMM-Newton study of the recently discovered plerion nebula in IC 433 is presented. In particular, we use the large effective area of the EPIC instrument to address the synchrotron burn-off effect in the nebula, to resolve its structure and measure the flux, and to constrain the thermal radiation of the central object in the nebula.