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S. Safi-Harb (NAS/NRC, NASA's GSFC), I.M. Harrus, R. Petre (NASA's GSFC), A.B. Koptsevich, D. Sanwal, G.G. Pavlov (Penn State)
G21.5-0.9 has been classified in the literature, prior to CHANDRA, as a plerionic (Crab-like) supernova-remnant. CHANDRA observations of this remnant show evidence of a shell-like component, giving the remnant a composite-type morphology, in X-rays. We present an X-ray study of G21.5-0.9 using CHANDRA data, along with archival ROSAT and ASCA data. The spectra of both the plerionic and shell-like components are well fitted with non-thermal models. The plerion is best fitted with a broken power law with NH=1.97\times1022 cm-2, a photon index \Gamma1=1.4, breaking at an energy Eb=2.7 keV, to \Gamma2=1.9. The corresponding unabsorbed flux is Fx(0.5--10 keV)=7.5\times10-11 erg~cm-2~s-1. We find no evidence of line emission from the shell-like component, using a 48.5 ksec observation with ACIS-S. The shell-like component is well fitted with a power law model with NH=1.90\times1022 cm-2, \Gamma=2.3, and an unabsorbed flux Fx(0.5--10 keV)=1.7\times10-11 erg~cm-2~s-1. A collisional equilibrium ionization (CEI) thermal model with solar abundances (such as Raymond-Smith) does not yield a good fit. Non-equilibrium ionization (NEI) models, which are more appropriate to model young supernova remnants, yield a very low ionization time-scale or alternatively unusually low abundances. Results of timing analysis using the CHANDRA HRC observations will be also presented.
We discuss the X-ray morphology of the remnant in correlation with high-resolution radio observations, and present the implications of our X-ray study on the parameters of the supernova remnant and its underlying engine.
S.S.--H. acknowleges support from the National Research Council.