Spatial and spectral profiles of O Ⅵ emission behind a shock wave on the northern edge of the Cygnus Loop were obtained with the Far Ultraviolet Spectroscopic Explorer. 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 km s~(-1) collisionless shock. Unlike faster shocks in SN 1006 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 Alfven Mach number, may control the degree of thermal equilibration. We combine the O Ⅵ observations with a low-resolution far-UV spectrum from the Hopkins Ultraviolet Telescope, an Hα image, and ROSAT PSPC X-ray data to constrain the preshock 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 preshock density is about twice the canonical value for the Cygnus Loop X-ray-emitting shocks.
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