No abstract

We have conducted a detailed analysis of the emission geometry of a handful of radio pulsars that have prominent, multiple-component profiles at meter wavelengths. From careful determination of the total number of emission components and their locations in pulse longitude, we find that all of the six pulsars show clear evidence for retardation and aberration effects in the conal emission beams. Using this information, coupled with a dipolar field geometry, we obtain estimates of the height and transverse location in the magnetosphere, for each of the emitting cones in these pulsars. These results support our earlier conclusions for PSR B0329+54 in that we find successive outer cones (in cases of multi-cone pulsars) being emitted at higher altitudes in the magnetosphere. The range of inferred heights is from ∼ 200 to ∼ 2200 km. The set of "active" field lines from which the conal emissions originate are located in the region from ∼ 0.22 to ∼ 0.74 of the polar cap radius. At the neutron star surface, these conal rings map to radii of a few to several tens of meters and the separation between successive rings is about 10 to 20 meters. We discuss the implications of these findings for the understanding of the pulsar emission geometry and for current theories and models of the emission mechanism.

We assume that relativistic sources moving along dipolar magnetic field lines emit curvature radiation. The beamed emission occurs in the direction of tangents to the field lines, and to receive it, the sight line must align with the tangent within the beaming angle 1= , where is the particle Lorentz factor. By solving the viewing geometry in an inclined and rotating dipolar magnetic field, we show that at any given pulse phase, the observer tends to receive radiation only from the specific heights allowed by the geometry. We find that outer conal components are emitted at higher altitudes compared to inner components, including the core. At any pulse phase, low-frequency emission comes from higher altitudes than high-frequency emission. We have modeled the emission heights of pulse components of PSR B0329+54 and estimated field line curvature radii and particle Lorentz factors in the emission regions.

The beamed radio emission from relativistic plasma (particles or bunches), constrained to move along the curved trajectories, occurs in the direction of velocity. We have generalized the coherent curvature radiation model to include the detailed geometry of the emission region in pulsar magnetosphere, and deduced the polarization state in terms of Stokes parameters. By considering both the uniform and modulated emissions, we have simulated a few typical pulse profiles. The antisymmetric type of circular polarization survives only when there is modulation or discrete distribution in the emitting sources. Our model predicts a correlation between the polarization angle swing and sign reversal of circular polarization as a geometric property of the emission process. Subject headings: polarization -pulsars: general -radiation mechanisms: non-thermal field in the directions parallel and perpendicular to the plane of particle trajectory, Gil and Snakowski (1990b) have developed a model to explain the depolarization and polarization angle deviations in subpulses and micropulses. Gil, Kijak and Zycki (1993) have modeled the single pulse polarization characteristics of pulsar radiation, and demonstrated that the deviations of the single pulse position angle from the average are caused by both propagation and geometrical effects. Mitra, Gil and Melikdze (2009) by analysing the strong single pulses with highly polarized subpulses from a set of pulsars, have given a very conclusive arguments in favor of the coherent curvature radiation mechanism as the pulsar radio emission mechanism.By analyzing the average pulse profiles, Radhakrishnan and Rankin (1990) have identified two most probable types of circular polarizations, namely, antisymmetric, where the circular polarization changes sense near the core region, and symmetric, where the circular polarization remains with same sense. They found that antisymmetric circular polarization is correlated with the polarization angle swing, and speculate it to be a geometric property of the emission mechanism. Han et al. (1998), by considering the published mean profiles, found a correlation between the sense of circular polarization and polarization angle swing in conal double profiles, and no significant correlation for core components. Further, You and Han (2006) have reconfirmed these investigations with a larger data. However, Cordes et al. (1978) were the first to point out an association between the position angle of the linear polarization and the handedness of the circular polarization.

The radiation by relativistic plasma particles is beamed in the direction of field line tangents in the corotating frame, but in an inertial frame it is aberrated toward the direction of rotation. We have revised the relation of aberration phase shift by taking into account of the colatitude of emission spot and the plasma rotation velocity. In the limit of small angle approximation, aberration phase shift becomes independent of the inclination angle α and the sight line impact angle β. However, at larger altitudes or larger rotation phases, the shift does depend on α and β. We have given an expression for the phase shift in the intensity profile by taking into account of aberration, retardation and polar cap currents. We have re-estimated the emission heights of the six classical pulsars, and analyzed the profile of a millisecond pulsar PSR J0437-4715 at 1440 MHz by fitting the Gaussians to pulse components. By this procedure we have been able to identify 11 emission components of PSR J0437-4715. We propose that they form a emission beam with 5 nested cones centered on the core. Using the phase location of component peaks, we have estimated the relativistic phase shift and the emission height of conal components. We find some of the components are emitted from the altitudes as high as 23 percent of light cylinder radius.