Abstract
THE radio source Cassiopeia A is well known to be associated with the remnants of a galactic supernova. Although optical studies are difficult because of heavy obscuration the object is seen to consist of a roughly circular nebulosity with an angular diameter of 4 min of arc. Minkowski1–3 has investigated the motions of the filaments and estimated the date of the explosion as AD 1702 ± 14. It is natural to expect a secular variation of flux density from so young an object and, following Minkowski's work, a mean rate of decrease of (1.06 ± 0.14) per cent per year was detected by Högbom and J. R. S.4 by means of observations in 1956 and 1960 combined with earlier observations by Ryle in 1948–9. Measurements at other frequencies have since been reported5–9 which confirm that there is a decrease but do not improve the accuracy of measurement.
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References
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SCOTT, P., SHAKESHAFT, J. & SMITH, M. Decrease of Flux Density of the Radio Source Cassiopeia A at 81.5 MHz. Nature 223, 1139–1140 (1969). https://doi.org/10.1038/2231139a0
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DOI: https://doi.org/10.1038/2231139a0
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