The extended X-ray halo of the Crab-like SNR G21.5-0.9
journal contributionposted on 24.10.2012, 09:21 by R. S. Warwick, R. G. Griffiths, N. Schurch, R. Willingale, J. -. P. Bernard, F. Bocchino, D. Lumb, A. Decourchelle, P. Ferrando, F. Haberl, A. M. Read, D. Schaudel, La Palombara N, S. Mereghetti, A. Tiengo
Recent XMM-Newton observations reveal an extended ( $r \approx 150''$) 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 $\Gamma = 1.63 \pm 0.04$ 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.