Pulsar timing methods for evaluating dispersion measure time series

¹ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
² Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, 09042 Monserrato (CA), Italy
³ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁴ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁵ Florida Space Institute, University of Central Florida, 12354 Research Parkway, Orlando, FL 32826, USA
⁶ Physics, School of Natural Sciences, Ollscoil na Gaillimhe – University of Galway, University Road, Galway, H91 TK33, Ireland
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁹ National Centre for Radio Astrophysics, Pune University Campus, Pune 411007, India
¹⁰ SETI Institute, 339 N Bernardo Ave Suite 200, Mountain View, CA 94043, USA
¹¹ School of Physics and Astronomy, Rochester Institute of Technology, Rochester, NY 14623, USA
¹² Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, Rochester, NY 14623, USA
¹³ National Research Council Postdoctoral Associate, National Academy of Sciences, Washington, DC 20001, USA resident at Naval Research Laboratory, Washington, DC 20375, USA
¹⁴ Space Science Division, Naval Research Laboratory, Washington, DC 20375–5352, USA
¹⁵ LPC2E – Université d’Orléans / CNRS, 45071 Orléans cedex 2, France
¹⁶ Observatoire Radioastronomique de Nançay (ORN), Observatoire de Paris, Université PSL, Univ Orléans, CNRS, 18330 Nançay, France

^★ Corresponding author; francesco.iraci@inaf.it

Received: 16 May 2024
Accepted: 19 October 2024

Abstract

Context. Radio pulsars can be used for many studies, including the investigation of the ionized interstellar medium and the solar wind via their dispersive effects. These phenomena affect the high-precision timing of pulsars and are among the main sources of noise in experiments searching for low-frequency gravitational waves in pulsar data.

Aims. In this paper, we compare the functionality and reliability of three commonly used schemes to measure temporal variations in interstellar propagation effects in pulsar timing data.

Methods. We carried out extensive simulations at low observing frequencies (100–200 MHz) by injecting long-term correlated noise processes with power-law spectra and white noise, to evaluate the robustness, accuracy, and precision of the following three mitigation methods: epoch-wise (EW) measurements of interstellar dispersion; the DMX method of simultaneous, piece-wise fits to interstellar dispersion; and DM GP, which models dispersion variations through Gaussian processes using a Bayesian analysis method. We then evaluated how reliably the input signals were reconstructed and how the various methods reacted to the presence of achromatic long-period noise.

Results. All the methods perform well, provided the achromatic long-period noise is modeled for DMX and DM GP. The most precise method is DM GP, followed by DMX and EW, while the most accurate is EW, followed by DMX and DM GP. We also tested different scenarios including simulations of L-band times of arrival and realistic DM injection, with no significant variation in the obtained results.

Conclusions. Given the nature of our simulations and our scope, we deem that EW is the most reliable method to study the Galactic ionized media. Follow-up works should be conducted to confirm this result via more realistic simulations. We note that DM GP and DMX seem to be the best-performing techniques in removing long-term correlated noise, and hence for gravitational wave studies. However, full simulations of pulsar timing array experiments are needed to support this interpretation.