Evolution of supernova remnants in the interstellar medium with a large-scale density gradient. I. General properties of the morphological evolution and X-ray emission

The large-scale gradient of the interstellar medium (ISM) density distribution essentially affects the evolution of Supernova remnants (SNRs). In a non-uniform ISM, the shape of SNR becomes essentially non-spherical, and distributions of gas parameters inside the remnant become strongly anisotropic. The well-known self-similar Sedov solutions may not be applied to modelling such non-spherical objects. Therefore we propose a new approximate analytical method for full hydrodynamical description of 3D point-like explosions in non-uniform media with arbitrary density distribution. On the basis of this method, we investigate the general properties of evolution of 2D non-spherical adiabatic SNRs in ISM with large-scale density gradient. It is shown that the real shape of adiabatic SNR becomes more non-spherical with age, but the visible shape remains close to spherical even for strong real asymmetry (ratio of maximal to minimal shock radii) and surface brightness contrast. It is shown also that the values of the parameters of X-ray radiation from the entire SNR (luminosity, spectral index) are close to those in the Sedov case with the same initial data. However, the surface distribution of the X-ray emission parameters is very sensitive to the initial density distribution around the SN progenitor. Therefore, the X-ray maps give important information about physical conditions inside and outside of the non-spherical SNR.