  G21.5-0.9 is a plerionic supernova remnant (SNR) used as a calibration target
for the Chandra X-ray telescope. The first observations found an extended halo
surrounding the bright central pulsar wind nebula (PWN). A 2005 study
discovered that this halo is limb-brightened and suggested the halo to be the
missing SNR shell. In 2010 the spectrum of the limb-brightened shell was found
to be dominated by non-thermal X-rays. In this study, we combine 15 years of
Chandra observations comprising over 1~Msec of exposure time (796.1~ks with the
Advanced CCD Imaging Spectrometer (ACIS) and 306.1~ks with the High Resolution
Camera (HRC)) to provide the deepest-to-date imaging and spectroscopic study.
The emission from the limb is primarily non-thermal and is described by a
power-law model with a photon index $\Gamma = 2.22 \, (2.04-2.34)$, plus a weak
thermal component characterized by a temperature $kT = 0.37\, (0.20-0.64)$ keV
and a low ionization timescale of $n_{e}t < 2.95 \times 10^{10}$ cm$^{-3}$s.
The northern knot located in the halo is best fitted with a two-component
power-law + non-equilibrium ionization thermal model characterized by a
temperature of 0.14 keV and an enhanced abundance of silicon, confirming its
nature as ejecta. We revisit the spatially resolved spectral study of the PWN
and find that its radial spectral profile can be explained by diffusion models.
The best fit diffusion coefficient is $D \sim 2.1\times 10^{27}\rm cm^2/s$
assuming a magnetic field $B =130 \mu G$, which is consistent with recent 3D
MHD simulation results.
