European EPTA consortium wins Royal Astronomical Society award

The Royal Astronomical Society has awarded the Group Achievement Award 2025 to a large-scale scientific project in which France has made a major contribution since its inception: the European Pulsar Timing Array (EPTA).

Photo de groupe des membres du consortium de l'European Pulsar Timing Array — **Group photo of the members of the European Pulsar Timing Array consortium, taken at the spring workshop held at the Brera Astronomical Observatory in Milan in March 2023.**

crédit: EPTA

EPTA was founded in the mid-2000s with the aim of revealing very low-frequency gravitational waves, thanks to the joint observation of very precise "cosmic clocks": pulsars. These are tiny but very dense neutron stars whose apparent beat (corresponding to stellar rotation) can vary, depending on their nature, from a few seconds to the extreme rate of over 700 revolutions in a single second: these ultra-fast-rotating pulsars are called "millisecond pulsars". By correlating the minute variations in the beats of these stars, it is possible to measure the passage of very low-frequency gravitational waves such as those originating from binary systems of supermassive black holes present at the center of galaxies.

In order to measure the minute variations in space-time produced by these very long waves, the EPTA consortium has brought together a dozen research institutes and over 80 researchers (in 2025, but many others have followed in their footsteps) who have observed and continue to observe dozens of pulsars for almost 25 years.

The very first EPTA observations were made in the 1990s, with the program really taking off in 2006, the year the collaboration was founded.

The campaigns were carried out by Europe’s largest and most powerful radio telescopes: Effelsberg (Germany), Jodrell Bank (UK), Nançay (France) and Westerbork (Netherlands), soon joined by the Sardinia radio telescope (Italy).

The quality of chronometric data acquired with a radio telescope, and therefore the sensitivity of temporal measurement series to gravitational waves, depends above all on outstanding instrumentation, which enables successive pulses received from pulsars to be processed and stacked in real time, then corrected for the imprint left by crossing the interstellar medium.

Birth and composition of the PTA-France team}

Following the initial impetus provided by Jean-François Lestrade and François Biraud (Paris Observatory) in the mid-1980s, Ismaël Cognard, now CNRS Research Director at LPC2E, has developed the various generations of pulsar instrumentation that have succeeded one another at the Nançay Radio Observatory since the 1990s.

The latest is based on a hybrid architecture using conventional processors, specialized integrated circuits (FPGAs) and, above all, super-powerful graphics cards enabling billions of operations per second. This instrumentation makes it possible to process a wide band of radio frequencies on the fly, obtaining highly stable and sensitive data. To date, it has provided almost 70% of EPTA’s chronometry data, with some 120,000 measurements between 2004 and 2024.

Joined in the early 2000s by Gilles Theureau (astronomer at Paris Observatory and researcher at LPC2E), then a specialist in large-scale extragalactic radio wave surveys, these two researchers founded what is now the PTA-France team.

This multi-site team brings together experts in multi-wavelength pulsar observations, such as Lucas Guillemot and Jean-Mathias Griessmeier, assistant astronomers at LPC2E, specialists in gravitational wave analysis and statistical processing, Stanislav Babak, research director at the APC laboratory (CNRS), Antoine Petiteau and Marc Besançon, researchers at IRFU (CEA), and experts in astrophysical and cosmological interpretation, Marta Volonteri, Director of Research at the IAP for the evolution of galaxies and their central black hole, Danièle Steer, Professor at the Physics Department of the ENS for the theories of cosmic strings and turbulence in the primordial universe, Chiara Caprini Professor at the University of Geneva and researcher at CERN for the role of primordial quantum fluctuations and cosmological inflation.

Several generations of young doctoral and post-doctoral researchers have also succeeded one another. some of whom have since moved on to other partner institutes in Europe and around the world:Grégory Desvignes, employed for 15 years by the Max Planck Institute in Bonn, Antoine Lassus, now a high school physics teacher, Siyuan Chen, recruited by Shanghai University, Aurélien Chalumeau, currently at the ASTRON Institute in the Netherlands, Mikel Falxa, now at the University of Milano Bicocca, Hippolyte Quelquejay-Leclere and Sara Manzini, currently doing their PhD at the APC laboratory.

After more than twenty years of accumulated data, the first results in terms of the detection of a low-frequency gravitational wave signal were presented in 2023, obtained from the analysis of a set of 25 pulsars.

A new version is underway, with an extended sample of 60 pulsars, which will enable us to better characterize the emission spectrum and improve the measurement of its spatial distribution on the celestial sphere.

Gilles Theureau explains: "The 2023 results are the fruit of a large-scale effort spanning several decades, during which it was crucial to pool the best of European instrumentation (including the large decimetric radio telescope at Nançay) and research, both in pulsar studies and in the astrophysics of ultra-long-period gravitational waves. The union of technical resources and skills was the fundamental ingredient.

This way of working is precisely why the Royal Astronomical Society has decided to award a Group Achievement Award, as the British institute itself explains, ahead of the official ceremony in a few months’ time: "One of EPTA’s strengths is its broad, diverse and egalitarian structure. By involving collaborators of different nationalities and backgrounds, and in particular by encouraging and supporting early-career researchers, EPTA is a model of international and intergenerational collaboration."

Birth and composition of the PTA-France team}

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