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C.E. Rakowski, J.P. Hughes (Rutgers, The State University of New Jersey), A Decourchelle (Service d'Astrophysique, CEA-Saclay)
The superb spatial and spectral resolution of the Chandra X-Ray Observatory make it uniquely suited to elucidate the physics of supernova remnant (SNR) shocks. In this paper, we measure the post-shock electron temperature and proper motion of SNR E0102.2-7219, specifically to address questions about the post-shock partition of energy among electrons, ions, and cosmic rays. For our X-ray expansion measurements, the spectral resolution of ACIS enables us to reconstruct how the Chandra observations would have appeared through the bandpasses and effective areas of the Einstein and ROSAT high resolution imagers, largely eliminating any concerns that measured expansion rates are due to spectral variations with position. Over the 20 year baseline thus obtained, we derive an expansion rate, 0.100% ±0.025% yr-1, for the X-ray remnant which corresponds to a ~6000 km s-1 velocity for the outermost blast wave. In the simplest, textbook case of the temperature discontinuity at a high Mach number shock, this velocity corresponds to a mean post-shock temperature of kTS = (3/16) \mu mp vS2 = 45+25-20 keV. In contrast, nonequilibrium ionization analysis of the Chandra spectrum of the immediate post-shock region indicates an electron temperature of 0.4-1~keV, at least 25 times smaller than the expected mean temperature. Furthermore, we can estimate the minimum post-shock electron temperature by assuming that the electron and ion temperatures initially differ by their mass ratio and then exchange energy only through Coulomb collisions. For the timescales indicated by our spectral fits, this would yield a minimum electron temperature of at least 2.5 keV, still significantly greater than our measured temperature. In order to reconcile our measured electron temperature with the shock velocity we are forced to conclude that a significant fraction of the shock energy has gone into accelerating cosmic rays. This interpretation is supported by the recent nonlinear shock acceleration models of Ellison 2000, which find that for nominal acceleration parameters and a Mach number consistent with E0102.2-7219's shock velocity, the mean post-shock temperature is ~1 keV. This work was supported in part by Chandra grant GO0-1035X.