Volume 624, April 2019
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||08 April 2019|
First detection of frequency-dependent, time-variable dispersion measures
Fakultät für Physik, Universität Bielefeld,
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
3 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia
4 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
5 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
6 SKA South Africa, The Park, Park Road, Pinelands 7405, South Africa
7 Department of Physics and Astronomy, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
8 GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
9 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
10 LPC2E – Université d’Orléans / CNRS, 45071 Orléans cedex 2, France
11 Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Université d’Orléans, OSUC, 18330 Nançay, France
12 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
13 Astronomisches Institut, Ruhr-Universität Bochum, 44780 Bochum, Germany
Accepted: 6 February 2019
Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies.
Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data.
Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented.
Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated.
Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited.
Key words: ISM: clouds / ISM: structure / pulsars: individual: PSR J2219+4754
© J. Y. Donner et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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