Measurements of the trigonometric parallax of pulsars

THE distances of pulsars from the Earth have been determined using the dispersion measure and an assumed constant value of the mean electron density through the Galaxy of about 0.03 cm −3 . Independent distance estimates are available for a number of distant pulsars from the measurement of the H I absorption in the pulsar radiation and these results are in general accord with the dispersion measure estimates over large distances through the Galaxy. There is some indirect evidence 1,2 that the mean electron density in the solar neighbourhood is typical of the rest of the Galaxy but all estimates of pulsar distances within 1 kpc of the Sun rely on this assumption. We report here measurements of trigonometric parallax of pulsars and from the distance estimates we obtain a value for the mean electron density within about 500 pc of the Sun.