Far Ultraviolet Spectra of a Non-Radiative Shock Wave in the Cygnus Loop

Spatial and spectral profiles of O VI emission behind a shock wave on the northern edge of the Cygnus Loop were obtained with the FUSE satellite. The velocity width of the narrowest O VI profile places a tight constraint on the electron-ion and ion-ion thermal equilibration in this 350 $\rm km s^{-1}$ collisionless shock. Unlike faster shocks in SN1006 and in the heliosphere, this shock brings oxygen ions and protons to within a factor of 2.5 of the same temperature. Comparison with other shocks suggests that shock speed, rather than Alfv\'{e}n Mach number, may control the degree of thermal equilibration. We combine the O VI observations with a low resolution far UV spectrum from HUT, an H$\alpha$ image and ROSAT PSPC X-ray data to constrain the pre-shock density and the structure along the line-of-sight. As part of this effort, we model the effects of resonance scattering of O VI photons within the shocked gas and compute time-dependent ionization models of the X-ray emissivity. Resonance scattering affects the O VI intensities at the factor of 2 level, and the soft spectrum of the X-ray rim can be mostly attributed to departures from ionization equilibrium. The pre-shock density is about twice the canonical value for the Cygnus Loop X-ray emitting shocks.