Changes in the angular size of the Crab pulsar

Related to the earlier posting about the gamma ray flare observed in the Crab remnant, it is worth mentioning the findings of astronomer A. G. F. Brown published in 1976:

Brown made interplanetary scintillation observations of the Crab pulsar at a radio frequency of 81.5 MHz and found that over a four year period between 1971 and 1975 the angular diameter of the Crab pulsar radio source increased over three fold from 0.2±0.1″ of arc to 0.7±0.1″ of arc.  The angular diameter of a pulsar’s radio image is determined by the amount of scattering its radio signal experiences as it encounters electrons in the interstellar medium.  A larger radio image diameter implies greater scattering which in turn implies greater interstellar electron concentration.  Brown ruled out changes in the solar plasma as being responsible for the change.  He also finds it unlikely that it is caused by changes in scattering in the immediate vicinity of the pulsar.

I would suggest that these changes in interstellar medium scattering are produced by the superwave that is now passing through the Crab nebula’s vicinity and which can change the electron density encountered along the line of sight to the Crab pulsar.

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