Chandra Observations of the Crab-like Supernova Remnant G21.5-0.9
  Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a compact central core and spectral variations indicative of synchrotron burn-off of higher energy electrons in the inner nebula. The central core is slightly extended, perhaps indicating the presence of an inner wind-shock nebula surrounding the pulsar. No pulsations are observed from the central region, yielding an upper limit of ~40% for the pulsed fraction. A faint outer shell may be the first evidence of the expanding ejecta and blast wave formed in the initial explosion, indicating a composite nature for G21.5-0.9. 
Patrick Slane (CfA), Yang Chen (CfA/Nanjing), Norbert S. Schulz (MIT),   Frederick Seward (CfA), John P. Hughes (Rutgers), a...