Astron. Astrophys. 323, 395-398 (1997)

1. Introduction

Polarization observations of pulsars provide much information about the physics of emission region. It is widely accepted that pulsar emission comes from the magnetic poles of neutron stars, and the behaviour of the position angle of linear polarization can be well interpreted by the rotating vector model (Radhakrishnan & Cooke 1969). However, the pulsar integrated profiles are diverse on the percentage of linear polarization from one pulsar to another and even from longitude to longitude of a given profile. Sometimes the linear polarization becomes to zero at a given longitude, so that one can see, almost certainly, that the smoothly-changing position-angle curve will suddenly jump almost at this longitude. One example is shown in Fig. 1. Similar phenomena can be found from polarization observations of PSR B1857-26 at 631 MHz, B0450-18 at 408MHz and B0450+55 at 409MHz (Lyne & Manchester 1988), B1937+21 at 1418MHz or higher frequencies (Thorsett & Stinebring 1990), B1929+10 at 430MHz and 1665MHz (Phillips 1990), B1855+09 at 1400MHz (Segelstein et al. 1986), B1839+09 at 1400MHz (Rankin, Stinebring & Weisberg 1989), B1601-52 at 660MHz (Qiao et al. 1995), etc.

Fig. 1. One example of the depolarization which results in polarization position-angle jumps. The polarization observation of PSR B1604-00 was done by Rankin (1988) at 430MHz.

Nonmonotonic and discontinuous rotation of polarization position angle observed from some pulsars is generally attributed to the occurrence of orthogonal or non-orthogonal modes of polarization in subpulses (Manchester, Taylor & Huguenin 1975; Cordes, Rankin & Backer 1978, Stinebring 1984a, b; Gil & Lyne 1995; Gil et al. 1992). Shier & Michel (1992) explain the discontinuities of position angle and multicomponentedness of pulsar profiles by involving the second emission hollow cone with orthogonal polarization. Certainly, the flips of the orthogonal or non-orthogonal emission modes of single-pulse emission can result in depolarization and position-angle discontinuities. However, it would be difficult to account the seperated polarization modes by Rankin (1988) for PSR B1604-00, which show that the position-angle curves of the core and conal components are seperated several tens degrees and that they sweep in different 'S' shapes. Similar phenomena can also be found from the integrated profile of PSR B0823+26 (Blaskiewicz, Cordes & Wasserman 1991) and its seperated polarization modes (Rankin & Rathnasree 1995).

In this paper, we investigate retardation effects as they effect the linearly polarized pulsar beam components of integrated pulse profiles, on the assumption that the core and conal beam components are radiated incoherently from different heights. We found that the phase shifts of beam centers of the different components could also cause the depolarization and position-angle jump(s) in integrated profiles.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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