2 Observed properties of thermal X-ray composites

RP98 prove that mixed-morphology SNRs create a separate morphology class since their properties distinguish these remnants from others. Having analysed X-ray data on a number of such SNRs, the authors found their two prominent morphological distinctions: a) the X-ray emission is thermal, the distribution of X-ray surface brightness is centrally-peaked or amorphous and fills the area within the radio shell and may reveal weak evidence of an X-ray shell, b) the emission arises primarily from the swept-up ISM material, not from the ejecta.

RP98 emphasize that, besides similar morphology, the sample of SNRs also has similar physical properties. Namely, a) the same or higher central density comparing with the edge, b) complex interior optical nebulosity, as in W28 and probably in 3C 400.2 (Long et al. 1991); c) higher emission measure $\int n_{\rm e}^2{\rm d}l$ ($n_{\rm e}$ is the electron number density, l is the length inside SNR) in the central region, as e.g. in 3C 391 (Rho & Petre 1996); d) X-ray surface brightness in the central region ( $r<0.2R_{\rm p},\ R_{\rm p}$ is the average radius of the projection), in general, exceeds the brightness near the edge ( $r>0.6R_{\rm p}$) 2-5 times, e) temperature profiles are close to uniform. As to the latter property, it should be noted that the temperature may decrease towards the centre, as in 3C 391 (Rho & Petre 1996); no strong evidence of increasing the temperature towards the centre has been found for all TXCs, but Cox et al. (1999) note that spectral hardness in W44 is greater in the centre, so the temperature might be higher in this region. A possible variation of temperature may be within factor 2, as in the case of W44 (Rho et al. 1994; Cox et al. 1999) or in W28 (Long et al. 1991).

7 objects from the list of 11 TXCs reveal observational evidence of an interaction with molecular clouds (RP98). Thus, ambient media in the regions of their localization are nonuniform and cause nonspherisity of SNRs. Observational evidence of cloud localization just on the line of sight for some of these SNRs also exists (e.g., Rho et al. 1994).