Recent XMM-Newton observations reveal an extended ( r \approx 150 ^ { \prime \prime } ) low-surface brightness X-ray halo in the supernova remnant G21.5-0.9 .
The near circular symmetry , the lack of any limb brightening and the non-thermal spectral form , all favour an interpretation of this outer halo as an extension 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 \pm 0.04 in the core to \Gamma = 2.45 \pm 0.06 at the edge of the halo .
Similar spectral trends are seen in other Crab-like remnants 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 .