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K.J. Borkowski, W.J. Lyerly, S.P. Reynolds, K.K. Dyer (NCSU)
X-ray spectra of supernova remnants (SNRs) are difficult to model because of nonequilibrium effects and the presence of nonthermal continua in many SNRs. As a rule, suitable nonequilibrium ionization (NEI) thermal models and nonthermal synchrotron models are not available to X-ray observers. This unsatisfactory situation has motivated us to reexamine the important issue of how to analyze SNR spectra in terms of relatively simple models, suitable for routine use by the astronomical community.
We focused our attention on the Sedov model. Our calculations include a realistic assumption of a partial electron heating at the shock and subsequent energy transfer from ions to electrons in Coulomb collisions. Spatially-integrated X-ray spectra can be well described by an approximate model involving appropriate electron temperature and ionization timescale distribution functions. This is a generalization of multitemperature equilibrium thermal X-ray models.
Constant temperature, single ionization timescale NEI models are not suitable approximations to Sedov models. Plane-parallel shocks provide a much better choice, particularly for high temperature Sedov models in ASCA and AXAF spectral ranges. The model parameters are post-shock mean and electron temperatures, Ts and Te, the shock ionization age \tau=net, and the elemental abundances. The full Sedov models must be used at lower shock temperatures. Their parameters are the current Ts and Te, the SNR ionization age \tau, and the elemental abundances.
The presence of synchrotron X-ray continuum in a SNR demands suitable models for electron acceleration and synchrotron emission. We developed realistic X-ray synchrotron models, taking into account all relevant physical processes such as finite acceleration time, synchrotron and inverse Compton losses, and escape of energetic electrons. Both thermal and nonthermal models have been implemented in our XSPEC package, finally providing a general tool for analysis of X-ray spectra of SNRs.
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