◼ ‘Pulsar Timing Arrays’ (PTAs) are arrays of pulsars [1] whose very stable rotation is used as a galactic scale gravitational wave detector. Pulsar signals are sensitive to gravitational waves with very low frequencies, in the order of the billionth of a Hertz. This technique extends the modes of detection of gravitational waves currently observed at high frequencies (hundreds of Hertz) by the ground-based LIGO/Virgo/Kagra detectors.
While ground-based detectors study short-duration collisions between stellar-mass black holes and neutron stars, PTAs allow the study of gravitational waves such as those emitted by pairs of supermassive black holes, which slowly approach each other as they spiral at the center of galaxies. The superposition of all the signals emitted by the total population of these binaries forms what is called a stochastic wave background.
The European Pulsar Timing Array (EPTA) is a European collaboration gathering about 40 scientists around the five largest European radio telescopes :
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Combined in LEAP (‘Large European Array for Pulsars’) mode, these EPTA radio telescopes form the equivalent of a 200-meter diameter, fully steerable dish, greatly improving the EPTA’s sensitivity to gravitational waves.
‘We can measure small fluctuations in the arrival times of the pulsars’ radio signals at Earth, caused by the spacetime deformation due to a passing-by very low frequency GW.’ explains Siyuan Chen, researcher at the Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (CNES / CNRS / University of Orléans) and the Station de Radioastronomie de Nançay (Observatoire de Paris - PSL / CNRS / University of Orléans), co-principal author of the study. In practice, these deformations appear as sources of very low frequency noise in the observed arrival time series of the radio pulses, a noise that is common to all pulsars of a PTA. However, the amplitude of this noise is extremely small (estimated to be tens to a couple hundreds of a billionth of a second) and if it were detected only for a particular pulsar, it could be attributed to many other possible effects.
◼ The European Pulsar Timing Array has thus identified a "promising signal", such as that produced by pairs of supermassive black holes in spiral phase, which could be related to the long-sought gravitational wave background at very low frequencies, on the order of billionths of a Hertz.
Although it cannot yet be declared a detection, the analysis represents an important step in the search for gravitational waves. The discovered signal is supported by an extremely detailed and unprecedented analysis based on two independent methodologies. To validate these results, several independent codes with different statistical methods were used, allowing to characterize and attenuate other spurious sources of noise and to search for the gravitational wave background itself.
Whatever the procedure used, the EPTA analysis clearly revealed the presence of a background whose spectral properties (i.e. the way the amplitude of the observed signal varies with frequency) are within the theoretical limits expected for the emission of gravitational waves by pairs of supermassive black holes.
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◼ This signal presents strong similarities with what was discovered at the same time by the work of competing teams. The EPTA consortium is indeed a founding member of the International Pulsar Timing Array (IPTA), which gathers worldwide efforts in this field. Independent data analyses by the other IPTA partners (in particular, the NANOGrav and PPTA experiments) have also revealed the presence of a similar common signal. "It has now become essential to independently apply multiple analysis algorithms and alternative models to increase our confidence in a possible future detection of the gravitational wave background," says Gilles Theureau, astronomer at Paris Observatory - PSL and member of the EPTA steering committee. The members of the IPTA are working together, comparing their data and methods to better prepare the next steps and draw more solid conclusions.
Einstein’s general relativity predicts a very specific relationship between the distortions in space-time experienced by radio signals from pulsars in different directions across the sky. Scientists call this the spatial correlation of the signal, or Hellings and Downs curve, after the two American researchers who first formulated this property in 1983. Its demonstration will make it possible to uniquely identify the observed background noise as being due to a signal of gravitational nature.
Collaboration
The French PTA team gathers researchers and PhD students from
- five laboratories (LPC2E, APC, LUTh, USN, DPhP/IRFU)
- and five academic institutions (Observatoire de Paris - PSL, CNRS, Université d’Orléans, Université de Paris and CEA). It relies in particular on the exploitation of the large decimetric radio telescope of Nançay (NRT), a national research infrastructure, financed by the Observatoire de Paris - PSL, the Institut National des Sciences de l’Univers of the CNRS (INSU) and the University of Orléans.
The project is in particular funded by the ANR "PTA-France" since 2019, and regularly supported by the Programme National des Hautes Energies (PNHE) et Programme National Cosmologie et Galaxies (PNCG) of the CNRS, and by the Scientific Council of Paris Observatory.
Bibliography
The study is published online in the article "Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array : Inferences in the stochastic gravitational-wave background search" by S. Chen et al, Monthly Notices of the Royal Astronomical Society, October 27, 2021.
DOI : 10.1093/mnras/stab2833
https://arxiv.org/abs/2110.13184
https://academic.oup.com/mnras/article/508/4/4970/6410749
Members of the scientific team in French institutes :
- Ismaël Cognard, research director at CNRS, LPC2E/CNRS
- Lucas Guillemot, Assistant Astronomer at the Observatoire des Sciences de l’Univers en région Centre (OSUC), LPC2E/CNRS
- Gilles Theureau, Astronomer at Observatoire de Paris - PSL, LPC2E/CNRS, USN/OP and LUTh/OP
- Stanislav Babak, Director of Research at CNRS, APC/CNRS
- Antoine Petiteau, researcher CEA-IRFU and APC/CNRS
- Siyuan Chen, post-doctoral researcher, LPC2E/CNRS and USN/OP
- Aurélien Chalumeau, PhD student, co-director of LPC2E/APC (DIM ACAV+)
- Anaïs Berthereau, PhD student LPC2E/CNRS (ANR PTA-France)
- Mikel Falxa, PhD student APC/CNRS (ANR PTA-France)
The European Pulsar Timing Array : http://www.epta.eu.org/
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[1] Pulsars are small stars of 20 to 30 km in diameter, as massive as the Sun and rotating very fast. The beam of radiation from the magnetic poles of pulsars rotates around their axis of rotation and is observed as regular pulses when they cross our field of view, like the beam of light from a distant lighthouse.
[2] The observation of pulsars has been for three decades now one of the key programs of the large decimetric radio telescope of the Station de radioastronomie de Nançay (Observatoire de Paris - PSL / CNRS / Université d’Orléans) which provided 60% of the EPTA data used in this study. With more than 2,500 telescope hours dedicated each year to this research program, the large radio telescope obtains some of the best pulsar timing observations in Europe (80 pulsars timed to an accuracy of 1.5 microseconds or better over 10 years) and, more importantly, the highest observing rate on a global scale