Large-scale non-uniformity of ISM density distribution in the SN neighborhood essentially affect the evolution of SNR. The shape of SNR becomes essentially non-spherical and distributions of parameters inside the remnant, as viewed from the center of explosion, are strongly anisotropic. We have proposed a new approximate analytical method for full hydrodynamical description of 3D point-like explosions in non-uniform media with arbitrary density distribution i.e., for cases, when well-known self-similar Sedov solutions are inacceptable. On the basis of it, we carry out the simulation of evolution of 2D non-spherical SNRs with special attention to their X-ray radiation. Since our aim in this paper was to investigate the role of density gradients in NSNR evolution, we restricted ourself with the ionization equilibrium case in the calculation of X-ray radiation. The role of electron conductivity, nonequilibrium and nonequipartition effects will be investigated elsewhere.
At first, we have investigated the shape of NSNR and brought out a remarkable fact of sphericity of visible shape of NSNR, even if the real deviation from sphericity is large. When the observed shape will differ from spherical by less than . Visual shape becomes noticeably non-spherical (more than 5% of visual assymetry) only for surface density contrast of order 100. Projection effects on the sky plane decrease the real shape anisotropy.
Therefore, we have calculated the equilibrium X-ray emission characteristics of NSNR. Again, the total (surface-integrated) parameters of X-ray radiation (luminosity, spectral index) are close to those in Sedov case with the same initial data. This is why many SNRs with apparently 2D anisotropic distribution of X-ray surface brightness are well described by the Sedov model.
Contrary to previous cases of weak dependence of shape and integral X-ray emission characteristics on density gradient, the distribution of X-ray emission characteristics along the surface of NSNR are very sensitive to an initial density distribution arround SN. Moreover, contrast of X-ray surface brightness S caused by density inhomogenity is most prominent, i.e., higher than density or temperature contrasts along the NSNR surface. For case of NSNR in Fig. 11 (), surface brightness contrast is but contrast of shock density equals 10 and of shock temperature equals 4 only.
Therefore, the surface brightness maps give the most promising information concerning the physical conditions inside and outside NSNR. It is important to note that typical contrasts of surface brightness caused by density inhomogenity (up to ) are considerably higher than those caused by nonequilibrium effects. Therefore, the role of density gradient is dominant in interpretation of NSNR observations.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999