From acollierastro.

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4-22. A pulsar emits a broad spectrum of electromagnetic radiation, which is detected with a receiver tuned to the neighborhood of f=80 MHz. Because of the dispersion in group velocity caused by the interstellar plasma, the observed frequency during each pulse drifts at a rate given by df/dt = -5 MHz/sec.

(a) If the interstellar magnetic field is negligible and w^2 much bigger than wp^2, show that :

df/dt ~ -c/x (f^3/fp^2)

where fp is the plasma frequency and x is the distance of the pulsar.

(b) If the average electron density in space is 0.2 cm^-3, how far away is the pulsar? (1 parsec =3×10^18 cm.)

Link to another coffee and the problem: https://www.youtube.com/watch?v=Z5aHMB4Tje4