News

Exciting pulsar timing array news about evidence for ultra-low-frequency gravitational waves. Have a look at this Nature article and this conversation article.

The PPTA

The Parkes Pulsar Timing Array (PPTA) project is a combined effort from astronomers across several institutions in which pulsars are observed using the Murriyang Parkes Radio Telescope, which is located near the town of Parkes in New South Wales, Australia. By monitoring such pulsars over many years it is possible to search for ultra-low-frequency gravitational waves, to develop pulsar-based time standards and to improve our understanding of solar system dynamics.

The Parkes Murriyang Radio Telescope situated in New South Wales, Australia. Credit: John Smith, CSIRO

The PPTA project was formed in 2004 and is the longest-running pulsar timing array project. The PPTA is a member of the International Pulsar Timing Array (IPTA), a global consortium in which we collaborate on pulsar timing array research. More details are available here about pulsars and the Murriyang Parkes radio telescope.

Pulsars and timing arrays

Pulsars are rapidly rotating neutron stars. Every time they rotate we detect a pulse of radio radiation. They are lighthouses in space! In the Parkes Pulsar Timing Array project we observe around 30 fast-rotating pulsars every 2 to 3 weeks with our ultra-wide-bandwidth receiver (operating between 704 and 4032 MHz). By studying the times of arrival of those pulses we can learn more about the pulsars themselves and how those pulses propagated through space. By comparing the arrival times for multiple pulsars we can use them as an array of sensors, for several different scientific studies.
A Pulsar Timing Array project uses radio telescopes observe a large number of millisecond pulsars. We measure the times of arrival of pulses from the pulsars with incredible precision. We then compare our model predictions for those arrival times with the measurements. Discrepancies can occur because of irregularities in the rotation of the pulsars, from variations in the interstellar medium, because of errors in our observatory clocks, or because of the presence of a gravitational wave background. With data sets collected over several years it is possible to distentangle these various phenomena.

Background image credit: Carl Knox, OzGrav

Results

A list of the PPTA publications can be obtained from the Astrophysics Data System (ADS). A few papers are highlighted below:

Search for gravitational waves: In this paper we describe our search for a background of nanohertz-frequency gravitational waves. We show the data are consistent with the background at approximately the 2sigma level.

The PPTA data releases: The PPTA has recently created its third data release. The data set will be publicly available from the CSIRO Data Archive and contains observations of 32 puslars with typical data spans of 18 years (with some increasing to 30 years with early observations). Our paper describes the creation of the data set, how to access the data set and the advantanges and challenges of using an ultra-wide-bandwidth receiver.

Millisecond pulsar noise analysis: We have analysed the noise present in the data sets for our millisecond pulsars in Reardon, Zic et al. (2023) using data from the PPTA third data release.

Wide-band timing of the Parkes Pulsar Timing Array UWL Data (2023): In this paper we presented the analysis of 3 years of PPTA data observed with our wide-band receiver system. This allowed us to obtain simultaneous measurements of pulse arrival times and dispersion measures.

Constraining ultralight vector dark matter with the Parkes Pulsar Timing Array second data release (2022): In this paper we used the second data release of the Parkes Pulsar Timing Array project to constrain the local dark matter density.

Evaluating the prevalence of spurious correlations in pulsar timing array data sets (2022): Here we explored how often false detections could be made in pulsar timing array studies.

We also note some exciting results from our IPTA partners. We are currently coordinating the publication of our most up-to-date data sets and our searches for the gravitational wave background. The results are expected to be made public in mid-2023. The IPTA has produced a checklist describing a formal process that will be followed to ensure PTA claims of detection are credible. The checklist is available from here.

About us

The PPTA team consists of astronomers around the World and contains experts in pulsars, gravitational waves, theoretical astrophysics, radio telescopes, software development and data analysis. The team currently consists of members from CSIRO, Swinburne University of Technology, Monash University, Macquarie University, University of Warsaw, Curtin University, Beijing Normal University, Hunan Normal University, Western Sydney University, US Naval Research Laboratory, Guangzhou University, Manly Astrophysics and the Auckland University of Technology.

The chair of the PPTA Steering Committee is George Hobbs (CSIRO), who can be contacted for further details. We are also linked to the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and members of the Gravitational Wave International Committee (GWIC) and the International Pulsar Timing Array (IPTA).