Pulsar polarisation below 200 MHz: Average profiles and propagation effects

¹ Max-Planck-Intitut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn Germany
e-mail: anoutsos@mpifr-bonn.mpg.de
² ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁴ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁵ Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁹ LPC2E – Université d’Orléans/CNRS, 45071 Orléans Cedex 2, France
¹⁰ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H30, PO Box 218, VIC 3122, Australia
¹¹ Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, 7535 Bellville, South Africa
¹² Astro Space Centre, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
¹³ Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, USR 704 – Univ. Orléans, OSUC, 18330 Nançay, France
¹⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁵ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), 44 Rosehill Street, Redfern, NSW 2016, Australia
¹⁶ Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile
¹⁷ FTI Beverly, Massachusetts Office, Frontier Technology, Inc., 100 Cummings Ctr #450G, Beverly, MA 01915, USA
¹⁸ AIM-Unite Mixte de Recherche CEA-CNRS, Université Paris VII, CEA Saclay, Service d’Astrophysique, 91191 Gif-sur-Yvette, France
¹⁹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²⁰ Laboratoire AIM, Université Paris Diderot, Paris 7/CNRS/ CEA-Saclay, DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
²¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²² Radio Astronomy Lab, UC Berkeley, CA, USA
²³ National RadioAstronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
²⁴ International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²⁵ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
²⁶ CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
²⁷ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
²⁸ Centrum Wiskunde & Informatica, PO Box 94079, 1090 GB Amsterdam, The Netherlands

Received: 20 October 2014
Accepted: 6 January 2015

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.