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Session 74 - Pulsars.
Oral session, Thursday, June 11

[74.03] Millisecond Pulsar Polarimetry: Decoding Neutron Star Magnetospheres

S. Sallmen, D. C. Backer (University of California at Berkeley)

A hollow-cone, rotating vector model explains the pulse profile morphology and polarization properties of long-period pulsars. In this model radio emission originates in the open-field line region above the polar cap (where the magnetic field lines do not close within the light cylinder), and is polarized along a direction fixed with respect to the local field. The open-field region is larger in millisecond pulsars, and emission in these ob jects may occur at altitudes which rotate at a speed closer to that of light. Relativistic effects and magnetic field distortions may therefore be more important. Pulse profile studies of millisecond pulsars indicate that the long-period pulsar classification system fails to account for the properties of these objects. Multi-frequency polarimetric observations provide important further information to compare with simple kinematic models of pulsar emission and magnetosphere geometry.

Polarization profiles with high temporal resolution are presented at three radio frequencies for several millisecond pulsars. Secure classification of the pulse profile morphology remains elusive for many objects. Pulse components are narrower than expected, and the spectral behaviour makes core and cone component identification uncertain. The component separations change very little with radio frequency, indicating that the emission region is very compact. These polarization profiles exhibit many of the features that are seen for normal pulsars. However, the radiation does not depolarize at high frequencies, and the distribution of slopes of the polarization position angle for millisecond pulsars has a narrower range than the distribution for normal pulsars. Although this effect has been attributed to magnetic field distortions, a simple geometric P^-1/2 scaling of the open field line region explains the observations. Variations in the intensity and polarization profiles are observed on long time scales which contrasts sharply with the very stable profiles of normal pulsars.

Program listing for Thursday