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Astron. Astrophys. 342, 474-479 (1999)

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1. Introduction

Pulsars are known for about the past three decades, and an enormous amount of effort has been devoted in understanding their emission process. There are models based upon plasma mechanisms (e.g. Asseo et al. 1990), maser mechanisms (e.g. Melrose 1992) and single particle approaches (e.g. Ruderman & Sutherland 1975). Some of them have succeeded in explaining radio pulse intensity, however, the polarization properties of individual pulses are hard to explain in the context of these models. Nevertheless, they do provide a basic understanding of the relevant physical processes.

The importance of polarization in the pulsar radio emission as a signature of emission mechanism is well known. To understand the average polarization angle swing seen in observations of radio pulsars, many attempts have been made to fit the observations with the rotating vector model (RVM) introduced by Radhakrishnan & Cooke (1969). The fits have been very successful in many cases. However, some pulsars do not fit with this interpretation (e.g. Manchester 1971; Rankin, Campbell & Backer 1974) and such discrepancies have been attributed to the occurrence of orthogonal polarization modes (e.g. Backer, Rankin & Campbell 1976; Gil & Lyne 1995; Gangadhara 1997). It becomes a very difficult task to fit the individual pulse polarization angle swings. The primary (or dominant) polarization mode is consistent with the RVM, and therefore reflects the geometry of magnetic field in the emission region (Gil et al. 1992). The polarization angle swings observed in micropulses and subpulses (Cordes and Hankins 1977) often do not fit with the RVM. As a physical explanation, Gangadhara (1997) has shown that such swings might arise due to the coherent superposition of orthogonal polarization modes.

It is well known that the individual pulses of many pulsars are highly polarized compared to the polarization of the integrated profiles (e.g. Manchester, Taylor & Huguenin 1975). Depolarization (less fractional polarization) is encountered in both average and single pulses, and the reasons given are: (i) propagation effects randomize the polarization angle (Manchester, Taylor & Huguenin 1975) (ii) the subpulse polarization state fluctuates from pulse-to-pulse (Cordes & Hankins 1977), (iii) frequent occurrence of orthogonal jumps (discontinuities of the polarization angle curve) (Lyne, Smith & Graham 1971; Manchester, Taylor & Huguenin 1975; Stinebring et al. 1984a&b; McKinnon & Stinebring 1996), (iv) systematic drift of subpulses (Taylor et al. 1971), (v) incoherent superposition of a large number of elementary emitters from different field lines (Gil, Kijak & [FORMULA]ycki 1993) and (vi) intrinsic to the emission process (Xilouris et al. 1994).

With the aim of understanding orthogonal polarization modes, the present work focuses on the role of instrumental time resolution on the polarization angle swing and the depolarization in individual pulses.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999