An international team of astronomers, including a large number of members of the European Pulsar Timing Array (EPTA) consortium, has announced the results of a comprehensive search for a background of low-frequency gravitational waves (GWs). These light-year-scale ripples, a consequence of general relativity, permeate all of spacetime and could originate from mergers of the most massive black holes in the Universe or from events occurring soon after the formation of the Universe in the Big Bang.
As a collaboration of teams of astronomers, data analysts and astrophysicists around the largest European radio telescopes: Effelsberg in Germany, Lovell in the UK, Nancay in France, Sardinia in Italy and Westerbork in the Netherlands and multiple affiliated research institutes, the European Pulsar Timing Array (EPTA) is part and contributes to the International Pulsar Timing Array (IPTA).
The IPTA, which joins the work of several astrophysics collaborations from around the world, recently completed its search for GWs in their most recent official data release, known as Data Release 2 (DR2). This data set consists of precision timing data from 65 millisecond pulsars – stellar remnants which spin hundreds of times per second, sweeping narrow beams of radio waves that appear as pulses due to the spinning – combined from independent data sets of the EPTA, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA), the three founding members of the IPTA.
One of the main expertise of the EPTA is in data combination. “The European Pulsar Timing Array itself is an international effort and we are used to combining data from up to five different radio telescopes and even observing simultaneously. This expertise has been very useful in the creation of the current data release”, says Dr. David Champion from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, member of the EPTA collaboration and co-author of the paper. Additionally, a large number of 42 pulsars and a long baseline of up to 18 years, accounts for a relative majority of the number of observations of the IPTA DR2. Dr. Caterina Tiburzi, from Astron in the Netherlands (also co-author and member of the executive committee of EPTA) adds: “Much of the used Bayesian methodology used was developed in the EPTA to establish upper limits for the GWB strength and to then understand the statistics of the emerging signal over the years; the EPTA maintains its own analysis codes”. The coordination and paper writing of the analysis of the IPTA DR2 has been led by a member of EPTA. Other EPTA members have also participated in the data analysis of the IPTA DR2.
This search includes an extensive comparison between individual data sets from the large regional scientific collaborations and the combined data set. The GW search of the IPTA DR2 has revealed strong evidence for a low-frequency signal detected by many of the pulsars in the combined data. Prof. Alberto Sesana, from the University of Milano Bicocca, EPTA member and co-author, explains: “The characteristics of this common-among-pulsars signal are in broad agreement with those expected from a GW “background” (GWB). This background is formed by many different overlapping GW signals emitted from the cosmic population of supermassive binary black holes (i.e., two supermassive black holes orbiting each other and eventually merging), analogous to background noise from the many overlapping voices in a crowded hall”. This result further strengthens the gradual emergence of similar signals that have been found in the individual data sets of the participating collaborations over the past few years.
“This is a very exciting signal! Although we do not have definitive evidence yet, we may be beginning to detect a background of gravitational waves,” says Dr. Siyuan Chen, a member of the EPTA and NANOGrav, and the leader of the IPTA DR2 search and publication. Dr. Boris Goncharov from the PPTA cautions on the possible interpretations of such common signals: “We are also looking into what else this signal could be. For example, perhaps it could result from noise that is present in individual pulsars’ data that may have been improperly modelled in our analyses.”
To identify the GWB as the origin of the low-frequency signal, the IPTA must also detect spatial correlations between pulsars; this means that each pair of pulsars must respond in a very particular way to the GWs, depending on their separation on the sky. Dr. Lucas Guillemot, from the University of Orléans in France, member of the EPTA collaboration and co-author of the paper, reports: “The response of each pair of pulsars to a gravitational wave signal displays a very specific pattern that is difficult to account for in any other way. A stronger signal than the one measured so far is needed to be able to detect this correlation”. Intriguingly, the first indication of a GWB would be a common signal like that seen in the IPTA DR2. Whether or not this spectrally similar low frequency signal is correlated between pulsars in accordance with the theoretical predictions for a gravitational-wave background will be resolved with further data collection, expanded arrays of monitored pulsars, and continued searches of the resulting longer and larger data sets.
Consistent signals like the one recovered with the IPTA analysis have also been published in individual data sets, more recent than those used in the IPTA DR2, produced from each of the three founding collaborations. Prof. Mike Keith, from the University of Manchester, in the UK, notes: “The fact that the same signal can already be seen in the currently used IPTA data set – with less data than those recently produced from each of the three founding collaborations – is a clear manifestation of the very promising potential of a full combination of the recent data”. For instance, new data from the MeerKAT telescope and from the Indian Pulsar Timing Array (InPTA), the newest member of the IPTA, will further expand future data sets. “The first hint of a GWB would be a signal like that seen in the IPTA DR2. Then, with more data, the signal will become more significant and will show spatial correlations, at which point we will know it is a GWB. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a GWB detection,” says Dr. Bhal Chandra Joshi, a member of the InPTA.
Recently, the EPTA produced a new dataset with 6 pulsars, extending the timespan of the observations to 24 years with more sensitive data, and analyzed it both for the search for a common signal using two independent pipelines as well as a single pulsar noise study. Dr. Delphine Perrodin, from INAF-Observatory of Cagliari in Italy, adds: “EPTA work is ongoing to expand the number to at least 25 pulsars; this extended EPTA dataset will be part of the next IPTA data combination.” Given the latest published results from the individual groups who now all can clearly recover the common signal, the IPTA is optimistic for what can be achieved once these are combined into the IPTA Data Release 3. Work is already ongoing on this new data release, which at a minimum will include updated data sets from the four constituent PTAs of the IPTA. We expect the analysis of the DR3 data set to be finished within the next few years. Dr. Maura McLaughlin of the NANOGrav collaboration says, “If the signal we are currently seeing is the first hint of a GWB, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future.”
Further Information
Original paper: https://academic.oup.com/mnras/advance-articleabstract/doi/10.1093/mnras/stab3418/6503453
DOI link: https://doi.org/10.1093/mnras/stab3418
arxiv link: https://arxiv.org/abs/2201.03980
International Pulsar Timing Array (IPTA): https://ipta4gw.org