Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977natur.268..317i&link_type=abstract
Nature, vol. 268, July 28, 1977, p. 317, 318. Research supported by the National Astronomy and Ionosphere Center.
Astronomy and Astrophysics
Astronomy
3
Crab Nebula, Pulsars, Wave Dispersion, Astronomical Models, Electron Density (Concentration), Interstellar Matter
Scientific paper
The contribution of the Crab Nebula to the dispersion measure of its pulsar, NP0532, is considered. The large contributions of the Crab Nebula suggested by some authors should be reflected in a negative slope in plateau regions of the dispersion measure resulting from dilution of the electrons in the nebulae. Possible reasons for the absence of this negative slope are examined. It is shown that cancellation of this negative slope by mechanisms increasing the dispersion measure is unlikely. If the dispersion plateau represents the base contribution of the nebula (in the line of sight to the pulsar) to the interstellar medium, the nebula contributes less than 1% of the total dispersion measure of NP0532, based on a model in which most of the contribution is assumed to arise from a thin spherical ionised shell. It seems likely that the pulsar is observed through a low-density hole in the nebula. The estimated lower limit of the mass of the nebula is 0.2 solar masses; the estimated average electron density of the interstellar medium in the direction of the Crab Nebula is 0.033/cu cm.
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