Polarization changes of pulsars due to wave propagation through magnetospheres

Wang, Chen; Lai, Dong; Han, J. L.

doi:10.1111/j.1365-2966.2009.16074.x

Cited by 75 publications

(89 citation statements)

References 45 publications

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“…Finally, a number of propagation effects in the pulsar magnetosphere have been proposed that act towards modifying the shape of the observed PA profiles, e.g. wave-mode coupling and the quasi-tangential propagation effect (Wang et al 2010). In addition, as was explained in Sect.…”

Section: Determining φmentioning

confidence: 99%

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

Noutsos

Sobey

Kondratiev

et al. 2015

A&A

110

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Aims. We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with the LOFAR high-band antennas. Based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium. Methods. The polarisation data presented in this paper have been calibrated for the geometric-projection and beam-shape effects that distort the polarised information as detected with the LOFAR antennas. We have used RM Synthesis to determine the amount of Faraday rotation in the data at the time of the observations. The ionospheric contribution to the measured Faraday rotation was estimated using a model of the ionosphere. To study the propagation effects, we have compared our low-frequency polarisation observations with archival data at 240, 400, 600, and 1400 MHz. Results. The predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. The derived spectra offer only partial support for the expected effects of birefringence on the polarisation properties, with only about half of our sample being consistent with the model's predictions. It is noted that for some pulsars these measurements are contaminated by the effects of interstellar scattering. For a number of pulsars in our sample, we have observed significant variations in the amount of Faraday rotation as a function of pulse phase, which is possibly an artefact of scattering. These variations are typically two orders of magnitude smaller than that observed at 1400 MHz by Noutsos et al. (2009), for a different sample of southern pulsars. In this paper we present a possible explanation for the difference in magnitude of this effect between the two frequencies, based on scattering. Finally, we have estimated the magnetospheric emission heights of low-frequency radiation from four pulsars, based on the phase lags between the flux-density and the PA profiles, and the theoretical framework of Blaskiewicz et al. (1991, ApJ, 370, 643). These estimates yielded heights of a few hundred km; at least for PSR B1133+16, this is consistent with emission heights derived based on radius-to-frequency mapping, but is up to a few times larger than the recent upper limit based on pulsar timing. Conclusions. Our work has shown that models, like magnetospheric birefringence, cannot be the sole explanation for the complex polarisation behaviour of pulsars. On the other hand, we have reinforced the claim that interstellar scattering can introduce a rotation of the PA with frequency that is indistinguishable from Faraday rotation and also varies as a function of pulse phase. In one case, the derived emission heights appear to be consistent with the predictions of radius-to-frequency mapping at 150 MHz, although this interpretation is subject to a number of systematic uncertainties.

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Section: Determining φmentioning

confidence: 99%

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

Noutsos

Sobey

Kondratiev

et al. 2015

A&A

110

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“…Here we did not consider the possible propagation effects, i.e., the wave mode coupling effect (Wang et al 2010) in the magnetosphere. Our calculations can explain some pulsar polarized emission, but it can not match various observation facts.…”

Section: Discussionmentioning

confidence: 99%

Curvature radiation in rotating pulsar magnetosphere

Wang

Han

2012

Proc. IAU

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We consider the curvature emission properties from relativistic particles streaming along magnetic field lines and co-rotating with pulsar magnetosphere. The co-rotation affects the trajectories of the particles and hence the emission properties, especially the polarization. We consider the modification of the particle velocity and acceleration due to the co-rotation. Curvature radiation from a single particle is calculated using the approximation of a circular path to the particle trajectory. Curvature radiation from particles at a given height actually contains the contributions from particles streaming along all the nearby field lines around the tangential point, forming the emission cone of 1/γ. The polarization patterns from the emission cone are distorted by the additional rotation, more serious for emission from a larger height. Net circular polarization can be generated by the density gradient in the emission cone. For three typical density models in the form of core, cone and patches, we calculate the polarization profiles for emission generated at a given height. We find that the circular polarization could have a single sign or sign reversal, depending on the density gradient along the rotation phase. The polarization profiles of the total curvature radiation from the whole open field line region, calculated by adding the emission from all possible heights, are similar to that from a dominating emission height. The circular polarization of curvature radiation has sign reversals in the patchy emission, while it has a single sign for the core emission, and is negligible for the cone emission.

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Dyks¹,

Rudak²

2014

Monthly Notices of the Royal Astronomical Society

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We present further development of a pulsar emission model based on multiple streams diverging away from the magnetic dipole axis, and forming azimuthally-structured fan-shaped beams. It is shown that this geometry, successfully tested on profiles with bifurcated features, naturally solves several classical pulsar problems and avoids some difficulties of the traditional nested cone/core model. This is best visible for profiles with several components, such as those of class T, Q and M, because they most clearly exhibit a range of effects previously interpreted within the conal model. In particular, with no reference to the flaring boundary of the polar magnetic flux tube, the stream model explains the apparent radius-to-frequency mapping (RFM), including its reduced strength for the inner pair of components. The lag of the central component (apparent 'core') with respect to the centroids of the flanking ('conal') components can also be naturally explained with no reference to emission rings located at disparate altitudes. The stream model also reveals why the millisecond pulsars, despite their more strongly flaring magnetic field lines, do not exhibit as strong RFM as the normal pulsars. The model is then successful in reproducing properties of so disparate objects as the M-class and millisecond pulsars, including some peculiarities of the latter. With no hesitation we, therefore, advance the view that pulsars have fan beams generated by outflowing streams, whereas the nested cone/core beams may well not exist at all.

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Polarization changes of pulsars due to wave propagation through magnetospheres

Cited by 75 publications

References 45 publications

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

Curvature radiation in rotating pulsar magnetosphere

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

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