The supernova remnant CTA1 (G 119.5+10.3): a study. of the breakout phenomenon

The supernova remnant (SNR) CTA1 has been observed with the Dominion Radio Astrophysical Observatory Synthesis Telescope, at 1420MHz and 408MHz, and with the Effelsberg 100 m telescope at 1420MHz. Angular resolution of the final maps is 1' and 3.5' at 1420 and 408MHz respectively. New HIRES infrared observations with a resolution of about 1' are also presented. Using those new observations, together with a new flux density measured from the 22.25MHz DRAO array and previously published values for the integrated flux density, we deduce an integrated spectral index of α=0.57+/-0.006 (where S_nu_{prop.to}ν^-α^). The new high-resolution radio maps confirm that, in addition to the bright radio arcs visible to the south and east, the SNR has a very substantial extension to the northwest, interpreted as the breakout of the SNR blast wave into a medium of lower density. Spatial variations are found in the spectral index distribution over the SNR, the diffuse emission to the northwest being generally of a steeper spectral index than emission associated with the brighter regions. Although this can be explained within the context of diffusive shock acceleration theory by variations in the Mach number of the SNR shock, it is not clear whether sufficiently large changes can occur following breakout. Faint emission is also present past the southern limb-brightened radio filaments. This could be the result of projection effects or due to electrons diffusing upstream ahead of the shock front with a mean free path of order 0.02pc. A break in the southeastern part of the radio shell is best explained by a density enhancement or cloud which has caused a drastic reduction in shock velocity possibly resulting in a decrease in the acceleration efficiency.