The extended X-ray halo of the Crab-like SNR G21.5-0.9

Recent XMM-Newton observations reveal an extended (r approximate to 150") l ow-surface brightness X-ray halo in the supernova remnant G21.5-0.9. The ne ar circular symmetry, the lack of any limb brightening and the non-thermal spectral form, all favour an interpretation of this outer halo as an extens ion of the central synchrotron nebula rather than as a shell formed by the supernova blast wave and ejecta. The X-ray spectrum of the nebula exhibits a marked spectral softening with radius, with the power-law spectral index varying from Gamma = 1.63 +/- 0.04 in the core to r = 2.45 +/- 0.06 at the edge of the halo. Similar spectral trends are seen in other Crab-like remna nts and reflect the impact of the synchrotron radiation losses on very high energy electrons as they diffuse out from the inner nebula. A preliminary timing analysis provides no evidence for any pulsed X-ray emission from the core of G21.5-0.9.