ROSAT PSPC observation of the northeast region of the vela supernova remnant .2. Spectral analysis with a nonequilibrium of ionization emission model

We have developed a grid of nonequilibrium of ionization (NEI) models that depends on the electron temperature and ionization time and that is suitable for use in fitting X-ray spectra. Using this grid, we have carried out a spatially resolved spectral analysis of a ROSAT Position Sensitive Proportional Counter (PSPC) observation of the northeast shock region of the Vela supernova remnant on an angular scale of 10' (similar to 1.0 pc at the distance of the remnant). The adopted model passes the X-2 test at the 95% confidence level for 45% of the considered spatial bins, giving unexpectedly low values of the ionization time (log tau similar to 0-1 yr cm(-3)). We discuss the possibility that this result may be due to a reduced contribution from a line emission, which may be related to the locking of metals in grains. We also find significant temperature gradients on the scale explored, and we argue that they are correlated to interstellar matter inhomogeneities encountered by the shock front. The emission from the remaining 55% of the bins is incompatible with the single-tau single-temperature NEI model, and a feasible description rests on the two-temperature collisional equilibrium of ionization model we have adopted in our previous analysis of the same data set. We tentatively interpret our results in terms of the superposition of shocks propagating in an inhomogeneous medium, which produces a multitemperature and multi-ionization stage plasma.