The Plerionic Supernova Remnant G21.5-0.9: In and Out
  The absence of a supernova remnant (SNR) shell surrounding the Crab and other plerions (pulsar wind nebulae) has been a mystery for 3 decades. G21.5-0.9 is a particularly intriguing plerionic SNR in which the central powering engine is not yet detected. Early Chandra observations revealed a faint extended X-ray halo which was suggested to be associated with the SNR shell; however its spectrum was nonthermal, unlike what is expected from an SNR shell. On the other hand, a plerionic origin to the halo is problematic since the X-ray plerion would be larger than the radio plerion. We present here our analysis of an integrated 245 ksec of archival Chandra data acquired with the High-Resolution Camera (HRC) and 520 ksec acquired with the Advanced CCD Imaging Spectrometer (ACIS). This study provides the deepest and highest resolution images obtained to date. The resulting images reveal for the first time 1) a limb-brightened morphology in the eastern section of the halo, and 2) a rich structure in the inner (40"-radius) bright plerion including wisps and a double-lobed morphology with an axis of symmetry running in the northwest-southeast direction. Our spatially resolved spectroscopic study of the ACIS-I data indicates that the photon index steepens with increasing distance from the central point source out to a radius of 40" then becomes constant at ~2.4 in the X-ray halo (for a column density N_H=2.2E22/cm^2). No line emission was found from the eastern limb; however marginal evidence for line emission in the halo's northern knots was found. This study illustrates the need for deep Chandra observations to reveal the missing SNR material in Crab-like plerions. 
Heather Matheson and Samar Safi-Harb