Pulsar velocities and the scaleheight of scattering in the Galaxy

Abstract

The velocities of pulsars can be obtained using two methods, one based upon angular proper motion measurements, the other upon the measurement of the velocity of the interstellar scintillation pattern of the pulsar radiation. The first method is more accurate but requires a long series of precise observations using a high-resolution radio interferometer, while the second can be carried out using a single telescope for perhaps an hour and can be applied to large numbers of pulsars. New pulsar proper motion determinations using MERLIN now permit an assessment of the accuracy of the scintillation measurements. We find large systematic differences, up to a factor of 5, between the velocities determined by the two methods. These can be understood in terms of the distribution of the scattering material within the Galaxy, permitting more reliable estimates of speeds from interstellar scintillation measurements. We also conclude that the scaleheight of the scattering material perpendicular to the galactic plane is an order of magnitude less than that of the dispersing electrons.