The Characteristics of Millisecond Pulsar Emission. II. Polarimetry

Abstract

We have made polarimetric monitoring observations of most of the millisecond pulsars visible from the northern hemisphere at 1410 MHz over a period of three years. 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 similar profile complexity, our 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 for normal pulsars. Moreover, the polarization characteristics suggest that millisecond-pulsar magnetospheres might not simply represent scaled versions of the magnetospheres of normal pulsars, supporting 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., J1730-2304). This suggests that while the basic emission mechanism remains insensitive to rotational period, the conditions that, according to the canonical pulsar model, regulate the radio emission, 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 due to 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 the 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 from future multifrequency observations. Fellow Subject headings: pulsars: millisecond -polarimetry -individual( J0613−

Coherently Dedispersed Polarimetry of Millisecond Pulsars

“…In Figure 7b we present the proÐle resulting from 330 minutes of observations acquired on 4 separate days and at a wide range of orbital phases ; these averaged data yield a maximum linear polarization of 9%. Our observations do not support the result of Xilouris et al (1998), who found a linear polarization fraction of 60%. The data of Xilouris et al (1998) were taken during a single observing session (K. M. Xilouris 1998, private communication) ; although we saw no similar results in our 4 daysÏ observations, it is possible that the linear polarization of 1744[24 is variable.…”

“…There is considerable overlap between the present set of pulsars and those studied by Xilouris et al (1998) at 1410 MHz using the 100 m telescope at E †elsberg, Germany ; our study provides a needed complement to these higher frequency observations. Recently, Sallmen (1998) has studied several of the pulsars presented here, conducting observations at the National Radio Astronomy Observatory, Green Bank, WV, and E †elsberg, Germany.…”

“…Sallmen (1998). Xilouris et al (1998) point out that equal position-angle slopes, though with opposite sign, for the main pulse and interpulse would argue for the classiÐcation of the pulsar as an orthogonal rotator. Unfortunately, while there is a small slope in the main pulse position-angle swing, the swing in the interpulse region is nearly Ñat, making this test impractical.…”

“…At 1414 MHz, we Ðnd that the linear intensity drops to very low levels. In contrast, Xilouris et al (1998) present a proÐle with reasonably strong linear polarization and a shallow position-angle swing. Sallmen (1998) reports a similar proÐle (strong linear intensity, shallow position-angle swing) derived from a single observation at 820 MHz at Green Bank.…”

“…To date, limited polarimetric studies of the faster millisecond pulsars (MSPs) have suggested that their pulse components do not Ðt easily into these classiÐcation schemes and frequently have more complex polarimetric proÐles than slow pulsars (Thorsett & Stinebring 1990 ;Navarro et al 1997 ;Xilouris et al 1998).…”