2019MNRAS.482.1031G

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 yr of Chandra observations comprising over 1 Ms of exposure time (796.1 ks with the Advanced CCD Imaging Spectrometer and 306.1 ks with the high-resolution camera) 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 Γ= 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 time-scale of n_et < 2.95 x 10¹⁰ 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-fitting diffusion coefficient is D ∼2.1×10²⁷ cm^2s-1^ assuming a magnetic field B = 130 µG, which is consistent with recent 3D MHD simulation results.