X-ray Emission from the Vela Snr Shock Region: Spectral Fitting with a Non-equilibrium Ionization Model

We report on the 5 0 scale spectral analysis of the X-ray emission from a region near the edge of the Vela SNR with a Non-Equilibrium Ionization (NEI) model. We have found signiicant variations of temperature, density, ionization time and interstel-lar absorption. We have identiied an overdense region with higher density and lower temperature than the surrounding medium. That can be interpreted as an ISM cloudlet recently shocked by the blast wave, not yet in thermal equilibrium. Our independent determination of the Vela SNR distance is in agreement with the most recent results based on ROSAT All-Sky Survey data. Our analysis indicates the occurrence of fast electron-ion energy equipartition behind the shock. 1. The X-ray Observation In order to perform spatially resolved spectral analysis, we have resampled a 4 ksec PSPC observation of the Vela SNR NorthEast region in 25 25 spatial bins with an eeective resolution of 5 0 (Fig. 1a). To avoid contamination from spurious events and electronic ghost images, we have discarded the photons at the end of OBI #1 (5.5% out of the data) and neglected SASS channels 1 and 2 in the spectral analysis. 2. The Non-Equilibrium Ionization (NEI) emission model We numerically solved the system of NEI diierential equations for each of the population fraction, with two main assumptions: (i) the convective term in the diiusion equation of the population fraction has been neglected. This is a good assumption in the Sedov uniform expansion phase, but it could break down in case of mass transfer between cloudlets and the environment. (ii) The electron temperature, T e , and density, n e , remain constant during the ionization process. This assumption is good within the shock region since the Sedov model predicts slow variations in regions of 1-2 parsecs behind the shock front of a Vela-like SNR (Cui & Cox 1992). We have used the Raymond-Smith code (Raymond & Smith, 1977; Raymond, 1989) for computing the grid of NEI X-ray spectra, which have been nally imported in XSPEC tabular form. 3. Results NEI ts were performed in each of the 273 spatial bins with more than 500 counts using the temperature T , the ionization time , the normalization factor and the