4 Summary and conclusions

The high spatial resolution of the EPIC camera on-board XMM-Newton has allowed us to look with unprecedented detail at the X-ray emission from the filled-center SNR 3C 58. We have presented soft (0.1-1 keV), medium (1-2 keV) and hard (2-10 keV) energy images, and we have found a close correlation of the soft image with radio emission at 1446 MHz. A quantitative study of the source profile shows the effect of synchrotron burn-off and indicates a different slope of the soft X-ray radial profile compared to that obtained in the hard band. Spatially resolved spectral analysis carried on in rings with $\Delta r = 8\hbox{$^{\prime\prime}$ }$ has allowed us to derive the relation between the spectral index and the distance from the core. In contrast with previous ASCA results, we have not found evidence for thermal black-body emission from a central source, and we have placed an upper limit to this component of $L_{\rm X}=1.8~10^{32}$ erg s^-1. This stringent upper-limit rules out thermal emission from the whole surface of the putative neutron star at the center of 3C 58, and also rules out the "outer-gap" model for hot polar caps of Cheng et al. (1986), unless the pulsar has very low rotational energy losses.

The large effective area of the XMM-Newton mirrors has allowed us to perform, for the first time, a spectral analysis of the outer edge of the 3C 58 X-ray nebula, excluding the inner and brighter regions. We have found that non-thermal emission is still responsible for most of the emission there, but a soft thermal component is required to better fit the spectrum below 1 keV. If represented with an optically thin emission model, this component gives kT=0.2-0.3 keV. If it is associated to the Sedov expansion of the 3C 58 shell, it is incompatible with the association between 3C 58 and SN 1181, unless there is strong deviation from electron-ion equipartition. If it is due to the expansion of the nebula into the inner core of moving ejecta, the X-ray spectra characteristics and the radio morphology of the outer nebula can be more easily reconciled.

Acknowledgements

We thank the whole XMM-Newton team for its effort in producing well-calibrated data at this early stage of the mission. We also thank the referee, R. Strom, for his useful suggestions. F. Bocchino acknowledges an ESA Research Fellowship and thanks R. Bandiera for helpful discussions.