Summary and conclusion

We have performed spatially resolved spectral analysis of a radius field in the NE shock region of the Vela SNR, using a pointed PSPC observation and a STNEI emission model we developed. We have derived maps of temperature and ionization time with a spatial resolution of . The maps show that variations of thermodynamical parameters exist on this scale. The hypothesis that the X-ray emission could be described everywhere by a single-tau single-temperature STNEI model with cosmic abundances is rejected, but 21 out of 48 spatial bins have acceptable values ( correspondent to the 95% confidence level). Most of these 21 bins shows unexpectedly low values of the ionization time ( in yr cm). Only 3 bins have acceptable and a reasonable value (> 1.0). Considering this results and those presented in Paper I, where we show the success of the 2T CIE model on the same data set, a multi-phase description of the emitting plasma, probably due to superposition of multiple shocks in an inhomogeneous and optically thin medium, is needed for a valid interpretation of the observed spectra. Low metal abundances cannot be excluded because this could explain the low values of the ionization time derived assuming cosmic abundances. Deep optical observations in H and other ``cooling" lines would be useful to confirm such a scenario, since ionizing material is expected to emit at H 6563 Å (Raymond, 1988) and cloudlets are expected to cool behind the shock and emit strongly in the optical band (McKee & Cowie, 1975). We have carried out such optical follow-up observations using the ESO-MPI 2.2m telescope at La Silla (Chile), which we shall report in a subsequent paper in this series.

Future planned extensions of this work will explore the abundances effect in the PSPC spectra, taking into consideration the recent findings of Vancura et al. (1994). We intend also to study the physical conditions arising from a shock expansion in a inhomogeneous medium using the 2D Palermo hydrodynamical code (Reale, Peres and Serio, 1991) coupled to an appropriate NEI emission code. We think that such an approach may contribute to understand better our data and, more in general, to interpret of the SNR X-ray data of ROSAT as well as of future detector with higher spatial and spectral resolution. Moreover, more realistic effects in the STNEI model itself as outlined in section 3.2 are still to be considered.

Acknowledgement: F. Bocchino wishes to thank stimulating discussion with O. Vancura and the Supernova Remnant Group at CfA in Cambridge, USA. The authors wish to thank the suggestion and comments from S. Serio, R. Pallavicini, F. Reale and G. Peres. We also thank the referee, D. Cox, for his constructive criticism about the manuscript. This work was partially supported by Ministero della Università e delle Ricerca Scientifica e Tecnologica, GNA-CNR and a contract of Italian Space Agency (ASI).

References

Aschenbach, B. 1993, Adv. Space Res., 13, 1245-1255

Bedogni, R., & Woodward, P. R. 1990, A&A, 231, 481

Bocchino, F. 1993, Thesis, University of Palermo

Bocchino, F., Maggio, A., & Sciortino, S. 1994 (Paper I), ApJ, 437, 209

Bocchino, F., Barbera, A., & Sciortino, S. 1996, Proceedings of ``RöntgenStrahlung from the Universe", Wurzburg (Germany), eds. H. U. Zimmermann, J. E. Trümper & H.York, MPE Report 263, 673

Bocchino, F., Maggio, A., & Sciortino, S. 1996, Proceedings of ``RöntgenStrahlung from the Universe", Wurzburg (Germany), eds. H. U. Zimmermann, J. E. Trümper & H.York, MPE Report 263, 235

Brinkmann, W. 1988, in Supernova Shells and Their Birth Events, Kundt W. ed., Springer - Verlag

Canizares, C. R. 1983, in Supernova Remnants and Their X-ray Emission, eds. Danzinger & Gorenstein, 205

Cox, D. P., Anderson, P. R. 1982, ApJ, 253, 268

Cui, W., & Cox, D. P. 1992, ApJ, 401, 206

Gorenstein, P., Harnden, F. R. Jr., & Tucker, W. H. 1974, ApJ, 192 661

Hamilton, A. J. S., & Sarazin, C. L. 1984, ApJ, 284, 601

Hamilton, A. J. S., Sarazin, C. L., & Chevalier R. A. 1983, ApJS, 51, 115

Hayashi, I., Koyama, K., Ozaki, M., Miyata, E., Tsunemi, H., Hughes, J. P., & Petre, R. 1994, PASJ,46, L121

Hughes, J. P., & Helfand, D. J. 1985, ApJ, 291, 544

Hughes, J. P., & Singh, K. P. 1994, ApJ, 422, 126

Hwang, U., Hughes, J. P., Canizares, C. R., & Markert, T. H. 1993, ApJ, 414, 219

Itoh, H. 1979, PASJ, 31, 541

Kahn, S. M., Gorenstein, P., Harnden, F. R. & Seward, F. D. 1985, ApJ, 299, 821

Lampton, M., Margon, B., & Bowyer, S. 1976, ApJ, 208, 177

Landau, L. D., & Lifshitz, E. M. 1974, Fluid Mechanics, Pergamon Press.

Lotz, W., 1967, ApJS, 14, 207

Masai, K. 1984, Ap&SS, 98, 367

Masai, K. 1994, ApJ, 437, 770

McKee, C. F., & Cowie, L. L. 1975, ApJ, 195, 715

McKee, C. F., & Hollenbach, D. J. 1980, Ann. Rev. A&A, 18, 219

Mewe, R., & Gronenschild, E. H. B. M. 1981, A&AS, 45, 11

Myata, E., Tsunemi, H., Pisarski, R., & Kissel, S. E. 1994, PASJ, 46, L101

Murphy, D. C., & May, J. 1991, A&A, 247, 202

Press, W. H., Flannery, B. P., Teukolsky, S. A., Vetterling, W. T. 1986, Numerical Recipes, Cambridge University Press

Raymond, J. C. 1988, in Hot Thin Plasmas in Astrophysics, R. Pallavicini ed, Kluwer Academic Publisher, 3-20

Raymond, J. C. 1989, private communication

Raymond, J. C., & Smith, B. W. 1977, ApJS, 35, 419

Reale, F., Peres, G., & Serio, S. 1990, Nuovo Cimento B, 105, 1235

Sedov, L. 1959, Similarity and Dimensional Methods in Mechanics, New York, Academic Press

Shull, M. J., van Steenberg, M. 1982, ApJS, 48, 95

Summers, H. P. 1974. Culham Laboratory Internal Memo IM-367, Culham Laboratory, Ditton Park, Slough, England.

Taylor, G. I. 1950, Proc. R. Soc. London, A201, 159 and 175

Vancura, O., Raymond, J. C., Dwek, E., Blair, W. P., Long, K. S., & Foster, S. 1994, ApJ, 431, 188

Vedder, P. W., Canizares, C. R., Market, T. H., & Pradhan, A. K. 1986, ApJ, 307, 269

Weiler, K. W., & Panagia, N. 1980, A&A, 90, 269

Weiler, K. W., & Sramek, R. A. 1988, Ann. Rev. A&A , 26, 295

Woltjer, L. 1972, Ann. Rev. A&A, 10, 129

Yamauchi, S., Ueno, S., Koyama, K., Nomoto, S., Hayashida, K., Tsunemi, H., & Asaoka, I. 1993, PASJ, 45, 795

Yamauchi, S., Kawai, N., & Aoki, T. 1994, PASJ, 46, L109

  
Table 1: Number of counts in each spatial bin of the X-ray image

  
Table 2: Statistical indetermination on best-fit parameter for the NEI model

  
Table 3: Best-fit parameters and derived quantities for the ``T3 bins"

Figure Captions

Fig. 1:

X-ray emissivity versus ionization time at various temperatures in the 0.1-2.0 keV band computed with our STNEI model, both unfolded (left) and folded with the PSPC instrumental response (right). The folded curves have been computed by generating the PSPC spectra for each model grid point and summing on SASS channels 3-30. Each curve is labelled with the corresponding temperature in keV.

Fig. 2:

From left to right: (a) Grey scale Map of the observed counts in the 0.1-2.0 keV band. The data have been sampled on a spatial grid reflecting the circular symmetry of the Vela SNR emission as a whole, and yielding few thousand counts per bin for an optimal parameter restoration with the STNEI model. (b) map. The grey scale top is at which is the threshold at the 95% confidence level. Spatial bins with are not displayed.

Fig. 3:

Top: (a): PSPC spectrum at position e6 with the correspondent best-fit STNEI spectrum, parameters, and residuals. We also report the confidence contours (68, 90 and 99% level) in the plane. The unfolded best-fit spectrum is shown on the right. Bottom: (b): Same as above but for bin c7.

Fig. 4:

PSPC spectrum at position d3 as an example of spectrum not fitted by our model.

Fig. 5:

From top left to bottom right: (a) Temperature map. (b) Ionization time map. Units and ranges are also indicated for each map.

  
Figure 5:

  
Figure:

  
Figure:

  
Figure:

  
Figure: