Radio Pulsar Profiles Research Articles

Pulsar polarization: a partial-coherence model

ABSTRACT The population of radio pulsars is observed to demonstrate certain polarization properties not explained by the conventional picture of pulsar polarization, namely frequency evolution of polarization, deviations of the linear polarization angle from a curve of geometric origins, and the presence of features in the circular polarization. We present the partial-coherence model as a way to explain the co-occurrence of these features and to provide an origin for circular polarization in radio pulsar profiles. We describe the mathematics of the model and demonstrate how it can explain these observed features, both on a population level and for the idiosyncrasies of individual pulsars. The partial coherence model can account for complex polarization behaviour, enabling improved access to information about pulsar geometries. We discuss the scientific implications of this for our understanding of pulsar radio emission and propagation.

Evidence for scattering of curvature radiation in radio pulsar profiles

ABSTRACT Radio pulsars exhibit several unexplained phenomena, in particular the average pulse profiles with the apparent core-cone structure and interesting frequency evolution. I show that they can be interpreted through essential geometric properties of the inverse Compton scattering. If the scattering occurs in a dipolar magnetosphere and the mean free path is long, a nested cone structure is expected with the cone size ratio of two-thirds, which is consistent with observations. Being a discontinuous process, the scattering is consistent with the discrete altitude structure of emission rings, as derived from aberration–retardation effects. Assuming that the upscattered signal is the curvature radiation (CR), one can interpret the observed bifurcated components (BCs) as a magnified microbeam of CR: the BCs are wide low-frequency CR microbeams that have been upshifted in frequency with their width preserved by beam-copying scattering in divergent magnetic field. The large flux of BCs is partly caused by compression of the full emitted spectrum into the narrow observed bandwidth, which explains why the frequency-resolved BCs have the frequency-integrated shape. The wide low-frequency microbeams can encompass large magnetospheric volumes, which considerably abates the requirements of the energy needed for coherency. The properties of BCs thus suggest that the observed modulated radio flux is strongly affected by the scattering-driven blueshift and spectral compression. The relativistic beaming formula (1/γ) is not always applicable, in the sense that it may not be directly applied to some blueshifted profile features.

A broad-band perspective on circular polarization in radio pulsar observations

AbstractWe investigate the broad-band behaviour of circular polarization in radio pulsar profiles and show the relationship between polarization fraction and what proportion of that polarization is circular, both across frequency, and for a large number of pulsars viewed collectively. The behaviour observed may be explained by pulsar polarization originating from the partially-coherent combination of two linearly-polarized orthogonal modes with different flux spectral indices. (See also the poster in the “supplementary information”.)

The geometry of a radio pulsar beam

Abstract Taxonomy of radio pulsar profiles is mostly based on a system of Ptolemaic artificiality, consisting of separated rings and a core, arbitrarily located at disparate altitudes in the magnetosphere. Diversity of observed profile shapes clearly exceeds the interpretive capability of such conal model. Moreover, bifurcated features observed in pulsar profiles imply a system of fan beams radially extending away from the dipole axis. The bifurcations can be understood as the imprint of the elementary radiation pattern of the long-sought radio emission mechanism, thus identifying the latter. Their size, however, is several times larger than implied by the curvature of magnetic dipole lines. Here, I show that the illusion of disconnected rings and the size of bifurcated features can be explained through a natural geometry that combines the properties of both the cone and the fan beam. It is a flaring spiral that makes several revolutions around the dipole axis on its way to leave the magnetosphere. Such geometry is consistent with a stream of outflowing and laterally drifting plasma. The bifurcated components are so wide because the curvature on such a spiral is larger than that of the dipolar magnetic field, and hence they are consistent with the extraordinary mode curvature radiation.

A model for distortions of polarisation-angle curves in radio pulsars

Some radio pulsar profiles (in particular those of millisecond pulsars contain wide emission structures which cover large intervals of pulse phase. Local distortions of an average curve of polarisation angle (PA) can be identified in such profiles, and they are often found to be associated with absorption features or narrow emission components. The features may be interpreted as a convolution of a lateral profile of an emitter with a microscopic radiation pattern of a non-negligible angular extent. We study a model which assumes that such an extended microbeam of the X-mode curvature radiation is spreading the radiation polarised at a fixed position angle within an interval of pulse phase. The model is capable of interpreting the strongly dissimilar polarisation of double notches in PSR B1821-24A (for which we present new polarisation data from the Nancay Radio Telescope) and PSR J0437-4715. It also explains a step-like change in PA observed at the bifurcated trailing component in the profile of J0437-4715. A generic form of the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of the second polarisation mode (O mode) can be magnified or transformed into a W- or U-shaped deflection of a total net PA. The model's efficiency in interpreting dissimilar polarisation effects provides further credence to the stream-based (fan-beam) geometry of pulsar emission. It also suggests that the microbeam width may not always be assumed negligible in comparison with the angular scale of emissivity gradients in the emission region.

Asymmetry of bifurcated features in radio pulsar profiles

High-quality integrated radio profiles of some pulsars contain bifurcated, highly symmetric emission components (BECs). They are observed when our line of sight traverses through a split-fan shaped emission beam. It is shown that for oblique cuts through such a beam, the features appear asymmetric at nearly all frequencies, except from a single `frequency of symmetry' nu_sym, at which both peaks in the BEC have the same height. Around nu_sym the ratio of flux in the two peaks of a BEC evolves in a way resembling the multifrequency behaviour of J1012+5307. Because of the inherent asymmetry resulting from the oblique traverse of sightline, each minimum in double notches can be modelled independently. Such a composed model reproduces the double notches of B1929+10 if the fitted function is the microscopic beam of curvature radiation in the orthogonal polarisation mode. These results confirm our view that some of the double components in radio pulsar profiles directly reveal the microscopic nature of the emitted radiation beam as the microbeam of curvature radiation polarised orthogonally to the trajectory of electrons.

A model for double notches and bifurcated components in radio profiles of pulsars and magnetars

Context.Averaged pulse profiles of three nearby pulsars: B1929+10, J0437-4715 and B0950+08 exhibit unusual “double notches”. These W-like looking features consist of two adjacent V-shaped dips that approach each other at increasing observation frequency roughly at a rate , where Δ is the separation between the notches' minima.

A model for ‘double notches’ in radio pulsar profiles

Aberration and retardation of the radiation within a pulsar's magnetosphere enable a single corotating absorbing region in the outer magnetosphere to give rise to a 'double notch' in the pulsar's observed radio profile. This effect requires that a pulsar has not only absorbing but also emitting regions at heights which are a substantial proportion of the light cylinder radius. The known presence of double notches in the weak secondary emission of three nearby pulsars suggests that this emission comes from open field lines well above their poles and that an obscuring region extending over no more than 5 percent of the light cylinder radius is located at or near the light cylinder.

Quasi-linear theory of Cherenkov-drift instability: Application for radio pulsars

We present a plasma model for formation of discontinuous position-angle curves observed in some radio pulsar profiles. The model is based on the polarization properties of radio waves excited by Cherenkov-drift instability develop- ing in pulsar magnetospheres. The radiation is confined in a cone of emission and consists of two orthogonally polarized waves. Examination of a quasi-linear stage of Cherenkov-drift instability shows that the excitation mechanism provides the modes with dierent spectral energy densities depending on the direction of k. As a pulsar rotates, an observer's sight-line track crosses dierent mode-predominated areas on the base of the emission cone. Transition of the sight-line track from one mode-predominated area to another results in the formation of pulsar polarization fluctuations observed in polarization angle distribution displays and average waveforms.

Geometrical effects on radio pulsar profiles and spectra

AbstractConsidering the geometry of the hollow-cone model of radio pulsar emission in detail, one is led to the conclusion that several well-known observed effects may be explained quite easily purely on geometrical terms. It is shown that a steepening of the spectrum (cut-off) may be expected for certain geometrical situations as well as variations in component spectra, e.g. steeper spectra for the central component. These effects appear in addition to the already existing physical variations with frequency.