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Session 16 - Radio Pulsars.
Display session, Monday, January 15
North Banquet Hall, Convention Center
Pulsars' emissions are highly polarized, allowing us to measure various properties of the spinning neutron stars. The most important is the position angle of the linearly polarized emission, which is related to the orientation of the emission beam on the sky.
The spin axis of an object will precess over time if the body is not distributed symmetrically with respect to the spin axis. This phenomenon is called free precession, and the resulting orientation changes will lead to secular changes in the polarized position angle. In a pulsar one would expect the cause of this precession to be an irregularity in the shape of the neutron star due to its cataclysmic birth.
The highly polarized nature of pulsar radio signals also leads to the most direct way of measuring the interstellar magnetic field, via Faraday rotation. By examining the variations in position angle over time, one can determine the strength of the interstellar magnetic field variations.
Over one hundred pulsars were observed at 21 cm with the Arecibo telescope in fifteen sessions during a four year period from 1989 to 1993. (See Weitz et al. contribution for details.) We determined full polarization parameters, including position angles, on all sufficiently strong pulsars. The position angles of several of the strongest pulsars were examined, and two of them, PSR B1929+10 and PSR B0540+23, were found to have steady position angles over short and long timescales. These two pulsars were used as position angle references for all of the other pulsars.
We performed a search for variations in position angle with time over all the sessions. If a change was seen, it could be explained either by precession or by variations in the magnetized interstellar medium between the pulsar and the Earth. We saw no evidence for a significant time variation in the position angle in any of approximately eighty pulsars on which we had adequate data for two or more sessions. We use these results to place upper limits on the amplitude of precession and of changes in the magnetized interstellar medium.
