The characteristics of millisecond pulsar emission. II. Polarimetry

We have made polarimetric monitoring observations of most of the millisecond pulsars visible from the northern hemisphere at 1410 MHz over a period of 3 yr. Their emission properties are presented here and compared with those of normal pulsars. Although we demonstrated in Paper I that millisecond pulsars exhibit the same flux density spectra and a similar profile complexity, the results presented here suggest that millisecond pulsar profiles do not comply with the predictions of classification schemes based on normal pulsars. The frequency development of a large number of millisecond pulsar profiles is abnormal when compared with the development seen in normal pulsars. Moreover, the polarization characteristics suggest that millisecond-pulsar magnetospheres might not simply represent scaled versions of the magnetospheres of normal pulsars, supporting the results of Paper I. However, phenomena such as mode-changing activity in both intensity and polarization are recognized here for the first time (e.g., for J1730-2304). This suggests that while the basic emission mechanism remains insensitive to rotational period, the conditions that regulate the radio emission in the canonical pulsar model might be satisfied at different regions in millisecond pulsar magnetospheres. At least three types of model have been proposed to describe the millisecond pulsar magnetospheres, ranging from distorted magnetic field configurations resulting from the recycled nature of these sources to traditional polar-cap emission and emission from outer gaps. A comparison of the predictions of these models with observations suggests that individual cases are better explained by different processes. However, we show that millisecond pulsars can be grouped according to common emission properties, a grouping that awaits verification with future multifrequency observations.