This is the second of a series of papers on a ROSAT PSPC observation of
the NE shock region of the Vela SNR. In Paper I (Bocchino, Maggio and
Sciortino 1994), we performed a spatially resolved spectral analysis
using Collisional Ionization Equilibrium (CIE) plasma emission
models: we showed that the X-rays originating from the shock front region
cannot be described in terms of a single-temperature model, and that a
second thermal component is required to give a statistically acceptable
description of the observed spectra. This seems to be the case down to the
smallest angular scale on which we were able to perform spectral
analysis (
, i.e. 0.5 pc at the distance of the SNR).
Accumulated evidence shows that the plasma of several SNR's is not in CIE. High spectral resolution X-ray observations of SNR's, such as those taken with the Einstein FPCS (for a review see Canizares 1983), have clearly shown, by detailed plasma diagnostics of He-like triplets, that the plasma dominating the emission, whether ejecta or swept-up interstellar medium, has not reached ionization equilibrium even in relatively old SNR. Apart from these high spectral resolution observations, which usually had low or none spatial resolution, the inclusion of relevant physical effects in the analysis of spatially resolved SNR X-ray emission, such as the non-equilibrium of ionization (NEI) or the ion/electron non-thermal equilibrium, has been traditionally prevented by the poor spectral resolution of imaging detectors (like Einstein IPC, EXOSAT LE+CMA, and HEAO-1 A2 experiments) as well as by the computational effort required to evaluate a model of the X-ray emission from a NEI optically thin plasma. Some efforts along this way have been made by Hamilton & Sarazin (1984), who created a grid of NEI emission spectra to be compared with spatially integrated remnant emission, by Hughes & Helfand (1985, hereafter HH85), who implemented a matrix algorithm approach for the solution of the population fraction equation in the NEI case, and by Masai (1984), who calculated the emission spectra (1-160 Å) in the transient phase of ionization. Masai (1994) gave a detailed description of non-equilibrium processes in SNR, including NEI. As a result, the NEI nature of the X-ray emission of many SNR's has been temptatively established by simultaneous measurements of the spatially averaged temperature and ionization time, which do not in general provide detailed spatial coverage and resolution (e.g. Hwang et al. 1993).
More recent accurate NEI analysis of SNR data have appeared in
the literature. For instance, Miyata et al (1994) reported on an ASCA
observation of a small (
arcmin
) region in the NE Rim
of the Cygnus Loop, and Hayashi et al (1994) examined the integrated
spectrum of E0102-72. The availability of the ROSAT and ASCA
missions to the scientific community now gives the opportunity to look
in greater detail and with a greater physical consistency at the SNR
X-ray emission, thanks to the improved spectral and spatial
resolution.
In fact, we are in the position to test the new picture being
developed since early Einstein FPCS observations: the SNR's
generally expand in a non uniform medium, the plasma is in NEI
condition, the post shock metal abundances
may not be the same as the cosmic values, and electrons and
ions may have different temperatures. These effects cannot be
overlooked when attempting to analyze spectra from
current detectors and will certainly be important with
future X-ray detectors on board SAX, Jet-X, AXAF and XMM.
Moreover, since the NEI emission models take into account an additional
physical effect with respect to CIE models, the results previously
obtained on SNR's with CIE models should in principle be revisited.
In this paper we present the results of a spatially resolved spectral
analysis of a 
square degree region in the NE of the Vela SNR,
using a NEI emission model. We take advantage of the angular
resolution of the ROSAT PSPC in order to resolve the temperature,
density and ionization time distributions of the remnant, which is
determined not only by the plasma dynamics but also by possible
inhomogeneities present in the ISM.
In this context, the lack of a NEI thermal plasma emission model in the most widely used software packages (PROS, EXSAS, XSPEC) for the analysis of the X-ray spectra is an obstacle to the widespread adoption of these more physical descriptions of the X-ray emission. To carry out this work, we have first developed a grid of NEI model spectra suitable to be used for fitting X-ray spectra. We have then used the grid to perform the spectral analysis of our data.
We will show that the single-ionization time, single-temperature NEI model we have developed (hereafter the STNEI model) provides a statistically acceptable description of the data for about 45% of the spatial bins analyzed. This result and the conclusions of Paper I tend to rule out the one-temperature nature of the X-ray emission of the Vela SNR shock region and leads naturally to a scenario with multiple shocks in an inhomogeneous medium, which could be quite common in middle aged SNR's.
Our paper is organized as follows: in section 2 the data are briefly reviewed, and in section 3 we discuss the basic assumptions for computing the STNEI plasma emission model we have adopted. In section 4 we describe the basic data analysis and in section 5 we give the results of the STNEI model fitting procedure. In section 6 we discuss the results of our analysis comparing them with previous results on the X-ray emission of the Vela SNR; finally, in section 7, we summarize our results.