XMM-Newton and SUZAKU detection of an X-ray emitting shell around the pulsar wind nebula G54.1+0.3

Context. X-ray observations have proven 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 information about the evolutionary stage of the embedded PWN. However, it is not clear why some PWNe are "naked". Aims. We perform an X-ray observational campaign targeting the PWN G54.1+0.3, the "close cousin" of the Crab Nebula, to try to detect the associated SNR shell. Methods. We analyze XMM-Newton and Suzaku observations of G54.1+0.3 to model the contribution of a dust scattering halo. Results. We detecte an intrinsic faint diffuse X-ray emission surrounding the PWN out to ∼ 6' (∼10 pc) from the pulsar, characterized by a hard spectrum, which can be modeled with either a power law (γ = 2.9) or a thermal plasma model (kT = 2.0 keV.) Conclusions. Assuming the shell to be thermal, we derive an explosion energy E = 0.5-1.6×10⁵¹ erg, a pre-shock ISM density of 0.2 cm^-3, and an age of ∼2000 yr. Using these results in the MHD model of PWN-SNR evolution, we obtain excellent agreement between the predicted and observed location of the shell and PWN shock.