Far-Ultraviolet Spectral Images of the Cygnus Loop

Abstract

We present far-ultraviolet (FUV) spectral images, measured at C IV λ1550, He II λ1640, Si IV+O IV] λ1400, and O III] λ1664, of the entire Cygnus Loop, observed with the SPEAR/FIMS instrument. The spatial distribution of FUV emission generally corresponds to a limb-brightened shell and is similar to optical, radio, and X-ray images. The features found in the present work include a carrot, diffuse interior, and breakout features, which have not been seen in previous FUV studies. Shock velocities of 140-160 km s-1 are found from the line ratio of O IV] to O III], which is insensitive not only to resonance scattering but also to elemental abundance. The estimated velocity indicates that fast shocks are widespread across the remnant. By comparing various line ratios with steady state shock models, it is also shown that resonance scattering is widespread.