Abstract
Abstract Observations of plerionic supernova remnant (SNR) G21.5-0.9 indicate an extended X-ray halo surrounding the bright central pulsar wind nebula (PWN), and that the photon index and the surface brightness of the SNR have been discovered to change with the increase of radial distance in the X-ray band. To explain the observational features of this source, a two-zone spatially dependent model with a slow diffusion in the PWN and a fast diffusion in the extended region is proposed in this paper. In the model, the evolution of electron inside the entire remnant is described with a particle transport equation under the assumption of a spherically symmetric system with dynamical evolution. The observed photon spectral energy distribution of the central PWN, as well as the radial profiles of the photon index and surface brightness observed in the X-ray band of the entire remnant, can be well reproduced in the framework of the model. The modeling results reveal that the X-ray halo of SNR G21.5-0.9 is mainly due to the synchrotron radiation of the accelerated electrons derived from the PWN shock, and the current diffusion coefficient in the extended region has a value of 1.7 × 1028 cm2 s−1 at an electron energy of 1 TeV, which is much larger than the obtained spatial averaged diffusion coefficient of 2.5 × 1025 cm2 s−1 in the central nebula.
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