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K.J. Borkowski (NCSU ), K.A. Arnaud, B. Dorman (NASA/GSFC), J.P. Hughes (Rutgers), C.L. Sarazin (UVA), R.A. Smith (SAO)
X-ray astronomy is advancing rapidly as data from the Chandra and XMM-Newton satellites become available. Among the brightest sources of X-ray emission are supernova remnants (SNRs). Their thermal X-ray emission cannot be analyzed with standard X-ray plasma codes, such as Raymond-Smith or MEKAL codes, because plasmas in SNRs are generally not in the equilibrium ionization. While there are various nonequilibrium ionization (NEI) codes available to researchers, each one of them has its own shortcomings, which results in a generally unsatisfactory situation.
We present new NEI code which addresses the problem just described. We integrated the most basic NEI models (such as constant temperature single ionization timescale models, plane-parallel shock models, and Sedov models with and without ion-electron equipartition) and the new Astrophysical Plasma Emission Code (APEC) within the XSPEC software package. The new NEI models provide significant improvements over the NEI models currently in use in XSPEC, and should allow for better modeling of existing and future X-ray data on SNRs. The most significant improvements include: radiative and dielectronic recombination (continua and lines), L-shell line emission, two-photon continuum emission, selected inner-shell processes, and inclusion of Ar. The much improved recombination and two-photon continua of heavy elements are particularly relevant for modeling low spectral resolution CCD observations of heavy-element dominated supernova ejecta in young SNRs. A more reliable modeling of X-ray grating spectra of SNRs also becomes possible, because of the improved quality of plasma diagnostic line ratios provided by APEC and the companion Astrophysical Plasma Emission Database (APED). We describe capabilities of the new package, and demonstrate how it may be used in practice on real Chandra datasets. We also discuss shortcomings of the current code when applied to extreme NEI conditions, particularly in heavy-element dominated supernova ejecta.
Support for this work was provided by NASA through grant NAG5-9490.