Crushing of interstellar gas clouds in supernova remnants II . X - ray emission

Context. X-ray observations of evolved supernova remnants (e.g. the Cygnus loop and the Vela SNRs) reveal emission originating from the interaction of shock waves with small interstellar gas clouds. Aims. We study and discuss the time-dependent X-ray emission predicted by hydrodynamic modeling of the interaction of a SNR shock wave with an interstellar gas cloud. The scope includes: 1) to study the correspondence between modeled and X-ray emitting structures, 2) to explore two different physical regimes in which either thermal conduction or radiative cooling plays a dominant role, and 3) to investigate the effects of the physical processes at work on the emission of the shocked cloud in the two different regimes. Methods. We use a detailed hydrodynamic model, including thermal conduction and radiation, and explore two cases characterized by different Mach numbers of the primary shock: M = 30 (post-shock temperature T psh ≈ 1.7 MK) in which the cloud dynamics is dominated by radiative cooling and M = 50 (T psh ≈ 4.7 MK) dominated by thermal conduction. From the simulations, we synthesize the expected X-ray emission, using available spectral codes. Results. The morphology of the X-ray emitting structures is significantly different from that of the flow structures originating from the shock-cloud interaction. The hydrodynamic instabilities are never clearly visible in the X-ray band. Shocked clouds are preferentially visible during the early phases of their evolution. Thermal conduction and radiative cooling lead to two different phases of the shocked cloud: a cold cooling dominated core emitting at low energies and a hot thermally conducting corona emitting in the X-ray band. The thermal conduction makes the X-ray image of the cloud smaller, more diffuse, and shorter-lived than that observed when thermal conduction is neglected.