Pulsar polarisation below 200 MHz: Average profiles and propagation effects

Abstract

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.

A LOFAR census of millisecond pulsars

“…Noutsos et al (2015) also report significant leakage to circular polarisation at large hour angles, however all our observations were carried out near transit where the leakage is minimal. Moreover, Faraday rotation from the ISM removes most of the contribution from linear polarisation in our large observing band.…”

“…The Stokes vector, (Noutsos et al 2015). Noutsos et al (2015) also report significant leakage to circular polarisation at large hour angles, however all our observations were carried out near transit where the leakage is minimal.…”

Wide-band, low-frequency pulse profiles of 100 radio pulsars with LOFAR

“…Parallel to this work, a similar one is being conducted on the evolution of the profiles of millisecond pulsars at low frequencies (Kondratiev et al 2016), and the spectral behaviour of the slow and recycled pulsars has been analysed (Hassall et al, in prep.). Complementary to our work is the study of the polarisation properties of pulsars (Noutsos et al 2015): the study of the polarisation properties can give constraints on the geometry of the pulsar emission and therefore on its height and on the intrinsic opening angle of the beam of the emission. Additionally, polarisation can help distinguish between orthogonal polarisation modes and therefore determine whether the observed widening of the profiles is caused by birefringence.…”

“…RFM predicts that the pulse profile will increase in width towards lower observing frequency, since emission will be directed tangentially to the diverging magnetic field lines of the magnetosphere that corotates with the pulsar. An alternative interpretation (McKinnon 1997) proposes birefringence of the plasma above the polar caps as the cause for broadening: the two propagation modes split at low frequencies, causing the broadening, while they stay closer together at high frequencies, causing depolarisation (this is investigated in the LOFAR work on pulsar polarisation, see Noutsos et al 2015).…”

Meterwavelength Single-Pulse Polarimetric Emission Survey

“…Polarization observations below 200 MHz are rare with the most notable exception of the recent study by Noutsos et al (2015) using the Low Frequency Array (LOFAR). Higher frequency polarization studies have used the Effelsberg radio telescope ) and the Parkes Telescope (Karastergiou et al 2005, Johnston & Weisberg 2006.…”