  Recent X-ray observations have proved to be very effective in detecting
previously unknown supernova remnant shells around pulsar wind nebulae (PWNe),
and in these cases the characteristics of the shell provide further clues on
the evolutionary stage of the embedded PWN. However, it is not clear why some
PWNe are still "naked". We carried out an X-ray observational campaign targeted
at the PWN G54.1+0.3, the "close cousin" of the Crab, with the aim to detect
the associated SNR shell. We analyzed an XMM-Newton and Suzaku observations of
G54.1+0.3 and we model out the contribution of dust scattering halo. We
detected an intrinsic faint diffuse X-ray emission surrounding a hard spectrum,
which can be modeled either with a power-law (gamma= 2.9) or with a thermal
plasma model (kT=2.0 keV.). If the shell is thermal, we derive an explosion
energy E=0.5-1.6x10^51 erg, a pre-shock ISM density of 0.2 cm^-3 and an age of
about 2000 yr. Using these results in the MHD model of PWN-SNR evolution, we
obtain an excellent agreement between the predicted and observed location of
the shell and PWN shock.

High Energy Astrophysical Phenomena