Abstract
THE Crab Nebula may account for as much as 35% of the dispersion measure, D, of its pulsar, NP0532 (ref. 1). Davidson and Terzian1 have pointed out that if this is so, and if the ionised material is distributed uniformly over the volume of the nebula, then the Crab's expansion2 of ā¼ 1,000 km sā1 should cause a decrease in D of about 0.07% per year. Isaacman and Rankin3 studied the dispersion measure over a 5-yr period, however, and observed no such decrease. From this, an upper limit to the Crab's contribution to D can be derived. It is much smaller than previously suspected.
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References
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ISAACMAN, R. NP0532 and a hole in the Crab Nebula. Nature 268, 317ā318 (1977). https://doi.org/10.1038/268317a0
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DOI: https://doi.org/10.1038/268317a0
