SIMBAD references

We present new observations of the Cygnus Loop at 408 MHz made with the Synthesis Telescope of the Dominion Radio Astrophysical Observatory. An analysis of this map with the 1420 MHz map of Leahy and Roger and the 2.695 GHz map of Keen and coworkers is carried out to determine radio spectral indices for various regions in the Cygnus Loop. We find several regions with significant spectral curvature between the ranges 408-1420 MHz and 1420-2695 MHz. The curvature is further confirmed by comparison with lower resolution 22 and 34 MHz maps. The spectral curvature shows a clear pattern related to specific features in the Cygnus Loop: the bright radio filaments all show negative curvature (steeper at higher frequency), and regions dominated by diffuse emission show positive curvature (flatter at higher frequency). The negative curvature is proposed to be due to absorption by a layer of thermal electrons with density 100-1000 cm^–3 associated with the sheets responsible for the filaments. The positive curvature is proposed to be due to superposition along the line of sight of steep and flat spectrum emission components.