1997A&A...326..811D

We present a detailed analysis of the northern part of the Cygnus Loop using the spectro-imaging capability of the PSPC and the superior spatial resolution of the HRI on board the ROSAT satellite. Large scale comparison between PSPC brightness and energy maps reveals that the bright emission structures are associated with soft emission, whereas the diffuse and faint regions correspond to harder one. Comparison with the optical image shows that the bright optical regions are associated with the soft and relatively bright X-ray emission, but not vice-versa. On the other hand, the hot tenuous X-ray regions do not show any optical emission. The interpretation of the north V-shaped structure in term of a bow-shock due to an interstellar cloud has been ruled out using the HRI, which shows filamentary emission along the western arm and diffuse emission with one bright knot on the eastern arm. Two close optical filaments underline the border of the shock front along the north rim as well as along the inner shell seen about 20 arcmin inside the remnant. A detailed spectral analysis focussing on specific homogeneous regions is presented. In bright soft regions, a two component thermal model is needed. Both equilibrium and non-equilibrium ionization models reproduce well the spectra, but only the latter is consistent with the derived parameters. The lower temperature can explain most of the observed O VI emission. In faint hot X-ray region, a single non-equilibrium ionization model is possible, providing a lower column density than is usually accepted (≃10²⁰cm^–2). We investigate how neglecting the non-equilibrium ionization effects as well as the abundance variations can affect the spectral interpretation.