Wide-band simultaneous observations of pulsars: disentangling dispersion measure and profile variations

¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL, UK
e-mail: tomehassall@gmail.com
² ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, 18330 Nançay, France
⁷ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, LPC2E UMR 7328 CNRS, 45071 Orléans Cedex 02, France
⁸ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
⁹ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
¹⁰ Université Paris 7 Denis Diderot and Service d’Astrophysique, UMR AIM, CEA Saclay, 91191 Gif-sur-Yvette, France
¹¹ Institut Universitaire de France, 75005 Paris, France
¹² Department of Physics & Astronomy, University of Sheffield, Sheffield S3 7RH, UK
¹³ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁴ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁵ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
¹⁶ Harvard-Smithsonian Center for Astrophysics, Garden Street 60, Cambridge, MA 02138, USA
¹⁷ Radio Astronomy Lab, UC Berkeley, CA, USA
¹⁸ International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
¹⁹ Space Telescope Science Institute, Baltimore, MD 21218, USA
²⁰ Centrum Wiskunde & Informatica (CWI), PO Box 94079, 1090 GB, Amsterdam, The Netherlands
²¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
²² LESIA, Observatoire de Paris, CNRS, UPMC Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
²³ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²⁵ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
²⁶ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
²⁷ Mt Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, Weston, A.C.T. 2611, Australia
²⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁹ NRAO Headquarters, 520 Edgemont Road, Charlottesville, VA 22903, USA
³⁰ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
³¹ Leibniz-Institut fr Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³² Center for Information Technology (CIT), University of Groningen, The Netherlands

Received: 6 February 2012
Accepted: 14 April 2012

Abstract

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40–190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 10⁵ across our observing band. We use this fact to constrain some of the properties of the interstellar medium along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays.