Nonradiative Shock in the Cygnus Loop: Implications for the Postshock Electron-Ion Equilibration

We present far ultraviolet spectra of a fast (∼ 300 km s) nonradiative shock in the Cygnus Loop supernova remnant. Our observations were performed with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, covering the wavelength range 905−1187 Å. The purpose of these observations was to estimate the degree of electron-ion equilibration by (1) examining the variation of O VI λλ1032, 1038 intensity with position behind the shock, and (2) measuring the widths of the OVI lines. We find significant absorption near the center of the O VI λ1032 line, with less absorption in O VI λ1038. The absorption equivalent widths imply an O VI column density greatly exceeding that of interstellar O VI along the Cygnus Loop line of sight. We suggest that the absorption may be due to resonant scattering by O VI ions within the shock itself. The widths of the O VI emission lines imply efficient ion-ion equilibration, in agreement with predictions from Balmer-dominated spectra of this shock.