Thermal X-ray composites as an effect of projection
  A new possibility to explain the nature of thermal X-ray composites (TXCs), i.e. a class of supernova remnants (SNRs) with a thermal X-ray centrally-filled morphology within a radio shell, as a projection effect of the 2- or 3-dimensional shell-like SNR evolved in a nonuniform medium with scale-height <10 pc is proposed. Both X-ray and radio morphologies, as well as the basic theoretical features of this kind of SNR and the surrounding medium, are considered. Theoretical properties of a shell-like SNR evolved at the edge of a molecular cloud correspond to the observed properties of TXCs if the gradient of the ambient density does not lie in the projection plane and the magnetic field is nearly aligned with the line of sight. So, at least a part of objects from the class may be interpreted within the framework of the considered effect. The proposed model suggests that SNRs with barrel-like radio and centrally-brightened thermal X-ray morphologies should exist. The model allows us to consider TXCs as prospective sources of proton origin gamma-rays. 
O. Petruk (IAPMM, Lviv, Ukraine)