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Astron. Astrophys. 342, 839-853 (1999)

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1. Introduction

This is the third paper of a series dedicated to ROSAT pointed PSPC observations of the Vela supernova remnant (SNR) shock region. In this series, we have taken advantage of the moderate PSPC spatial and spectral resolution to carry out a spatially resolved spectral analysis of the X-ray emission in the 0.2-2.0 keV band. The region we have analyzed has an area of [FORMULA] square degrees, 8% of the total extension of the Vela SNR X-ray emission as measured by the ROSAT All-Sky Survey (Aschenbach 1993). In Bocchino et al. (1994, Paper I), we showed that a single-temperature thermal emission model in Collisional Equilibrium of Ionization (hereafter the 1T CIE model), such as the one described in Raymond & Smith (1977), does not give an adequate description of the data. This is an important result because, even though the plasma in SNR's is expected to be in Non Equilibrium of Ionization (NEI, Itoh 1979), the 1T CIE model was extensively used in the past to describe the X-ray emission of the Vela SNR (Kahn et al. 1985). At the same time, we pointed out that the spectral features of the X-ray emission change noticeably at the explored scale length of [FORMULA] pc. In Paper I, we also noted that a two-temperature Raymond & Smith (1977) thermal emission model in CIE condition (2T CIE) fits satisfactory the data, but, following Itoh (1979), we suggested that this result could be attributed to NEI effects and to the moderate PSPC spectral resolution; for this reason, we did not interpret the fitting results as an evidence of two distinct components in the emitting plasma. In Bocchino et al. (1997, Paper II), we developed and tested a single-temperature - single-ionization time NEI emission model for fitting the PSPC data (hereafter the STNEI model), and we found that in order to achieve a satisfactory fit, the ionization time had to be unreasonably low ([FORMULA] yr cm-3). The difficulty to find a physical explanation for this result led us to reconsider multi-component emission models as a feasible description of the Vela SNR X-ray emission. In particular, we argued that the failure of a single-temperature model (either in CIE or in NEI conditions) and the success of a two-temperature model in describing the data might reflect the intrinsic nature of the post-shock plasma, and may indicate, for instance, the presence of inhomogeneities in the swept up interstellar medium. In this paper, we intend to explore this possibility, and we shall show that this interpretation is not only consistent with the observational evidences, but also it provides us with insights on the local ISM structure.

The paper is organized as follows. In Sect. 2 we present the data, in Sect. 3 we outline the results of the application of the two-temperature model, while in Sect. 4 we discuss the interpretation of the thermal components and we estimate the remnant and ISM characteristic parameters. We summarize our achievements in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999