Secular changes within Cassiopeia A at 5 GHz-I

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Abstract
A detailed comparison of radio maps of Cassiopeia A at 5 GHz, obtained with the Cambridge 5-km radio telescope in 1974 and 1978, is described. Accurate proper motions and brightness changes are presented for 342 distinct radio peaks, ranging in angular size from 1 arcmin for fragments of the intense ring of emission to less than the beam size of 2.0 arcsec × 2.3 arcsec for compact structure. The radio peaks are moving systematically away from a point offset by 2.1 arcsec N, 14.9 arcsec W from the expansion center of the fast optical filaments. The time-scale for the expansion of both compact and broad-scale radio structure is confirmed to be three times that of the fast optical filaments, and several intense compact radio peaks situated near the periphery of the radio plateau have individual outward motions significantly slower than is predicted for swept-up matter downstream of a blast wave propagating in a uniform external medium. Proper motions are independent of the sign of brightness changes. Unlike the fast optical filaments, the radio peaks have non-systematic components of motion of typically 0.1 arcsec yr−1 which are not due to measurement errors or changes in shape.

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