Far Ultraviolet Spectroscopy of a Nonradiative Shock in the Cygnus Loop: Implications for the Postshock Electron-Ion Equilibration

Abstract

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 Angstroms. The purpose of these observations was to estimate the degree of electron-ion equilibration by (1) examining the variation of O VI 1032, 1038 intensity with position behind the shock, and (2) measuring the widths of the O VI lines. We find significant absorption near the center of the O VI 1032 line, with less absorption in O VI 1038. 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.

arXiv comment4 pages, 2 figures, text of poster paper presented at the 11th Annual Astrophysics Conference in Maryland, to appear in Young Supernova Remnants, ed. by S. S. Holt and U. Hwang (AIP)