The extended X-ray halo of the Crab-like SNR G21.5-0.9
Haberl, F.; Bernard, J. -P.; Read, A. M.; Tiengo, A.; Mereghetti, S.; La Palombara, N.; Bocchino, F.; Decourchelle, A.; Warwick, R. S.; Ferrando, P.; Lumb, D.; Willingale, R.; Griffiths, R. G.; Schaudel, D.; Schurch, N.
United Kingdom, France, Netherlands, Germany, Italy, Spain
Abstract
Recent XMM-Newton observations reveal an extended (r ~ 150'') 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 +/- 0.04 in the core to Gamma = 2.45 +/- 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.