Polarization Characteristics of Pulsar Profiles

Polarization profiles of several hundred pulsars have been published recently (eg. Gould & Lyne 1998, GL98; Weisberg et al. 1999). In this report, we summarize the characteristics of circular and high linear polarizations of pulsar profiles, based on all previously published data. Han et al. (1998) systematically studied the circular polarization of pulsar profiles. They found that circular polarization is common in profiles but diverse in nature. Circular polarization is about 9% in average, weaker than linear polarization (15%) in general. We emphasize the following points about circular polarization (CP): (1). CP not unique to core emission: One misleading concept is that circular polarization alway accompanies core emission. It is generally strongest in the central or 'core' regions of a profile, but is by no means confined to these regions. Circular polarization has been detected from conal components of many pulsars, for example, conal-double pulsars. (2). Sense reversal not unique to core: Circular polarization often changes sense near the middle of the profile. But sense reversals have been observed at other longitudes, e.g. conal components of PSRs B0834+06, B1913+16, B2020+28, B1039−19, J1751−4657, and B0329+54 in abnormal mode. (3). No PA correlation for core emission: There is no correlation between the sense of the sign change of circular polarization and the sense of variation of linear polarization angle (PA), in contrast to earlier conclusions on this issue. (4). Correlation for cone-dominated pulsars: We found a strong correlation between between the sign of PA variation and sense of circular polarization in conal-double pulsars, with right-hand (negative) circular polarization accompanying increasing PA and vice versa. No good examples contrary to this trend have been found. (5). Variation with frequency: Circular polarization generally does not vary systematically with frequency. Multifrequency profiles of many pulsars show similar CP across a wide frequency range (eg. PSRs B0329+54, B0525+21). However, we have found several examples with significant variations (Table 5 of Han et al 1998). For example, PSR B1749−28 now has been confirmed by the data of GL98 to change from dominant right hand (" − ") CP at low frequencies to a sense change " −/+ " at high frequencies, while PSRs B1859+03 and B1900+01 change from " + " to " +/− " .