We present the analysis of archival X-ray observations of the supernova remnant ( SNR ) G21.5 - 0.9 .
Based on its morphology and spectral properties , G21.5 - 0.9 has been classified as a Crab-like SNR .
For that reason , it was chosen as a Chandra calibration target .
In their early analysis of part of these calibration data , Slane et al .
( 2000 ) discovered a low-surface-brightness , extended emission .
They interpreted this component as the blast wave formed in the supernova ( SN ) explosion . XMM-Newton observations ( Warwick et al .
2000 ) revealed the non-thermal nature of this emission suggesting that it is instead an extension of the synchrotron nebula .
In this paper , we revisit the Chandra analysis using new calibration data , improving the statistics by a factor of 2 .
We also include ROSAT and ASCA observations .
Our analysis confirms the non-thermal nature of the extended emission .
ACIS-S images indicate that this component is not limb-brightened , and it shows knotty structures and a bright filament 2 ^ { \prime } north of the center .
We find no evidence of line emission from any part of the remnant .
We can reject a collisional equilibrium ionization ( CEI ) thermal model at solar abundances , and non-equilibrium ionization ( NEI ) models ( such as a plane-parallel shock model with different ionization ages and constant-temperature , or an NEI model with a single ionization age and a constant temperature ) .
The entire remnant is best fitted with a power law model with a photon index steepening away from the center .
The total unabsorbed flux F _ { X } ( 0.5 – 10 keV ) is 1.1 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } with an 85 % contribution from the 40 ^ { \prime \prime } radius core .
Timing analysis of the High-Resolution Camera ( HRC ) data failed to detect any pulsations .
We put a 16 % upper limit on the pulsed fraction .
We derive the physical parameters of the putative pulsar and compare them with those of other plerions ( such as the Crab and 3C 58 ) .