Far-Ultraviolet Spectra of a Nonradiative Shock Wave in the Cygnus Loop
Abstract
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 Alfvén 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.
Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by Johns Hopkins University under NASA contract NAS 5-32985.- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2003
- DOI:
- 10.1086/345741
- arXiv:
- arXiv:astro-ph/0210572
- Bibcode:
- 2003ApJ...584..770R
- Keywords:
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- ISM: Individual: Name: Cygnus Loop;
- Shock Waves;
- ISM: Supernova Remnants;
- Ultraviolet: ISM;
- Astrophysics
- E-Print:
- To appear in Astrophysical Journal, Vol. 584, Feb. 20 2003