A LOFAR census of millisecond pulsars

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: kondratiev@astron.nl
² Astro Space Centre, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁸ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H30, PO Box 218, VIC 3122, Australia
⁹ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), The University of Sydney, 44 Rosehill Street, Redfern, NSW 2016, Australia
¹⁰ SKA Organisation, Jodrell Bank Observatory, Lower Withington, Macclesfield, Cheshire, SK11 9DL, UK
¹¹ School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ LPC2E – Université d’Orléans/CNRS, 45071 Orléans Cedex 2, France
¹⁴ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, OSUC, 18330 Nançay, France
¹⁵ Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁶ Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, 7535 Bellville, South Africa
¹⁷ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
¹⁸ NAOJ Chile Observatory, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁹ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius ( Cagliari), Italy
²⁰ CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
²¹ Laboratoire AIM (CEA/IRFU – CNRS/INSU – Université Paris Diderot), CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
²² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²⁴ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France

Received: 12 August 2015
Accepted: 19 October 2015

Abstract

We report the detection of 48 millisecond pulsars (MSPs) out of 75 observed thus far using the LOw-Frequency ARray (LOFAR) in the frequency range 110–188 MHz. We have also detected three MSPs out of nine observed in the frequency range 38–77 MHz. This is the largest sample of MSPs ever observed at these low frequencies, and half of the detected MSPs were observed for the first time atfrequencies below 200 MHz. We present the average pulse profiles of the detected MSPs, their effective pulse widths, and flux densities and compare these with higher observing frequencies. The flux-calibrated, multifrequency LOFAR pulse profiles are publicly available via the European Pulsar Network Database of Pulsar Profiles. We also present average values of dispersion measures (DM) and discuss DM and profile variations. About 35% of the MSPs show strong narrow profiles, another 25% exhibit scattered profiles, and the rest are only weakly detected. A qualitative comparison of the LOFAR MSP profiles with those at higher radio frequencies shows constant separation between profile components. Similarly, the profile widths are consistent with those observed at higher frequencies, unless scattering dominates at the lowest frequencies. This is very different from what is observed for normal pulsars and suggests a compact emission region in the MSP magnetosphere. The amplitude ratio of the profile components, on the other hand, can dramatically change towards low frequencies, often with the trailing component becoming dominant. As previously demonstrated this can be caused by aberration and retardation. This data set enables high-precision studies of pulse profile evolution with frequency, dispersion, Faraday rotation, and scattering in the interstellar medium. Characterising and correcting these systematic effects may improve pulsar-timing precision at higher observing frequencies, where pulsar timing array projects aim to directly detect gravitational waves.