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

¹ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
² Institute d'Astrophysique Spatiale, Orsay, France
³ Space Science Department, ESTEC, 2200 AG Noordwijk, The Netherlands
⁴ Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁵ Max-Planck-Institut für extraterrestrische Physik, 85740, Garching, Germany
⁶ Istituto di Fisica Cosmica "G. Occhialini", CNR, 20133 Milano, Italy
⁷ XMM-SOC, Villafranca Satellite Tracking Station, 28080 Madrid, Spain

Corresponding author: R. S. Warwick, rsw@star.le.ac.uk

Accepted: 9 November 2000

Abstract

Recent XMM-Newton observations reveal an extended () 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 in the core to 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.