Dispersion measure variability for 36 millisecond pulsars at 150 MHz with LOFAR

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: jdonner@mpifr-bonn.mpg.de
² Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³ ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁴ Gravitational Wave Data Centre, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia
⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia
⁶ LPC2E – Université d’Orléans / CNRS, 45071 Orléans cedex 2, France
⁷ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Université d’Orléans, OSUC, 18330 Nançay, France
⁸ SKA South Africa, The Park, Park Road, Pinelands 7405, South Africa
⁹ Department of Physics and Astronomy, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
¹⁰ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
¹¹ Technische Universität Berlin, Institut für Geodäsie und Geoinformationstechnik, Fakultät VI, Sekr. H 12, Straße des 17. Juni 135, 10623 Berlin, Germany
¹² GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
¹³ SKA Organisation, Jodrell Bank, Macclesfield SK11 9FT, UK
¹⁴ Astro Space Centre, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia
¹⁵ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA 6102, Australia
¹⁶ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁷ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹⁸ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching b. München, Germany
²⁰ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
²¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 24 September 2020
Accepted: 15 November 2020

Abstract

Context. Radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. Variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (PTAs) that aim to detect nanohertz gravitational waves.

Aims. We aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations.

Methods. We use the pulsar timing technique to obtain highly precise dispersion measure (DM) time series. Our dataset consists of observations of 36 millisecond pulsars, which were observed for up to 7.1 yr with the LOw Frequency ARray (LOFAR) telescope at a centre frequency of ~150 MHz. Seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence.

Results. We achieve a median DM precision of the order of 10⁻⁵ cm⁻³ pc for a significant fraction of our sources. We detect significant variations of the DM in all pulsars with a median DM uncertainty of less than 2 × 10⁻⁴ cm⁻³ pc. The noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1–10 μs at 1.4 GHz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 μs or better that PTAs aim for. We found no evidence for a dependence of DM on radio frequency for any of the sources in our sample.

Conclusions. The DM time series we obtained using LOFAR could in principle be used to correct higher-frequency data for the variations of the dispersive delay. However, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (ToAs) at 1.4 GHz or a high DM precision at low frequencies, but not both, due to spectral properties. Combining the higher-frequency ToAs with those from LOFAR to measure the infinite-frequency ToA and DM would improve the result.