Understanding the effects of geometry and rotation on pulsar intensity profiles

Abstract

We have developed a method to compute the possible distribution of radio emission regions in a typical pulsar magnetosphere, taking into account the viewing geometry and rotational effects of the neutron star. Our method can estimate the emission altitude and the radius of curvature of particle trajectory as a function of rotation phase for a given inclination angle, impact angle, spin period, Lorentz factor, field line constant and the observation frequency. Further, using curvature radiation as the basic emission mechanism, we simulate the radio intensity profiles that would be observed from a given distribution of emission regions, for different values of radio frequency and Lorentz factor. We show clearly that rotation effects can introduce significant asymmetries into the observed radio profiles. We investigate the dependency of profile features on various pulsar parameters. We find that the radiation from a given ring of field lines can be seen over a large range of pulse longitudes, originating at different altitudes, with varying spectral intensity. Preferred heights of emission along discrete sets of field lines are required to reproduce realistic pulsar profiles, and we illustrate this for a known pulsar. Finally, we show how our model provides feasible explanations for the origin of core emission, and also for one‐sided cones which have been observed in some pulsars.

The Frequency-dependent Behavior of Subpulse Drifting. I. Carousel Geometry and Emission Heights of PSR B0031–07

“…Let L represent the line whose footpoint has the smaller colatitude. For a dipolar field, one expects from geometric arguments (neglecting rotation effects on curvature, as discussed in Gangadhara 2007 andThomas et al 2010) that the lower frequency emission will be observed from L and the higher frequency emission from H, and that the emission occurs at a greater altitude for L than for H. From Eq. (15),…”

“…Note that although the direction of the subpulse phase track shift is fixed by the carousel geometry, the magnitude of the shift depends on r , which may well be a strong function of pulse phase (e.g. Gangadhara 2004;Thomas et al 2010). This can introduce further distortions in the shape of the subpulse phase track as it appears at different frequencies.…”

“…( 26) invalid. This idea has been explored by Thomas & Gangadhara (2007) and Thomas et al (2010), who showed how the curvature of the field lines on the leading side is augmented by rotation effects, while on the trailing side the curvature is diminished. A more complete modelling of P 2 vs frequency for B0031−07 that takes rotation-induced curvature is beyond the scope of this analysis-we only note here that this effect (or some other effect that we haven't considered) may be necessary for a complete understanding of the observed frequency-dependent behaviour of P 2 in B0031−07 if AR effects are the dominant cause.…”

On the Geometry of Curvature Radiation and Implications for Subpulse Drifting

“…In this case, the observed emission pattern for curvature radiation from a single particle can be calculated, and scaled up by the square of the local plasma density at each emission site. Thomas et al (2010) describe one possible method for doing such a calculation, although it should be noted that they incorporate the phase-shifting effects of aberration and retardation post hoc. To our knowledge, no similar method exists for estimating intensity profiles assuming other possible mechanisms, such as maser mechanisms (Ginzburg & Zhelezniakov 1975) or plasma wave instabilities (Cheng & Ruderman 1977).…”

The origin of the frequency-dependent behaviour of pulsar radio profiles

“…1 of Karastergiou & Johnston (2007). A carefull recent study of the sign reversing circular polarisation is presented in Gangadhara (2010), Wang et al (2012), and Wang et al (2010). Moreover, Michel's fan beams were very elongated (∆θ ∼ 1 rad), and more complicated than the single-parameter conal beams, which seemed to be so well designed to match the imposing flaring polar tube.…”