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G21.5-0.9 is a plerionic supernova remnant (SNR) used as a calibration targetfor the Chandra X-ray telescope. The first observations found an extended halosurrounding the bright central pulsar wind nebula (PWN). A 2005 studydiscovered that this halo is limb-brightened and suggested the halo to be themissing SNR shell. In 2010 the spectrum of the limb-brightened shell was foundto be dominated by non-thermal X-rays. In this study, we combine 15 years ofChandra observations comprising over 1~Msec of exposure time (796.1~ks with theAdvanced CCD Imaging Spectrometer (ACIS) and 306.1~ks with the High ResolutionCamera (HRC)) to provide the deepest-to-date imaging and spectroscopic study.The emission from the limb is primarily non-thermal and is described by apower-law model with a photon index $\Gamma = 2.22 \, (2.04-2.34)$, plus a weakthermal component characterized by a temperature $kT = 0.37\, (0.20-0.64)$ keVand 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-componentpower-law + non-equilibrium ionization thermal model characterized by atemperature of 0.14 keV and an enhanced abundance of silicon, confirming itsnature as ejecta. We revisit the spatially resolved spectral study of the PWNand 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 3DMHD simulation results.

The deepest Chandra X-ray study of the plerionic supernova remnant G21.5$-$0.9