Detection of a Strong Candidate Signal for the Stochastic Gravitational Wave Background at NHz Frequencies

The European Pulsar Timing Array (EPTA) is a scientific collaboration bringing together teams of astronomers around the largest European radio telescopes, as well as groups specialized in data analysis and modelling of gravitational wave signals. Dr. John Antoniadis, a researcher who recently joined IA-FORTH, is a pulsar expert and active member of EPTA for the past several years.  Recently, the EPTA published a detailed analysis of a candidate signal for the since-long sought gravitational wave background due to in-spiraling supermassive black-hole binaries. Although a detection cannot be claimed yet, this represents another significant step in the effort to finally unveil gravitational waves at very low frequencies, of order one billionth of a Hertz. In fact, the candidate signal has emerged from an unprecedented detailed analysis and using two independent methodologies. Moreover, the signal shares strong similarities with those found from the analyses of other teams.

The results were made possible thanks to the data collected over 24 years with five large-aperture radio telescopes in Europe. They include the 100-m Radio Telescope of the Max Planck Institute for Radio Astronomy near Effelsberg in Germany, the 76-m Lovell Telescope in Cheshire/United Kingdom, the 94-m Nançay Decimetric Radio Telescope in France, the 64-m Sardinia Radio Telescope at Pranu Sanguni, Italy and the 16 antennas of the Westerbork Synthesis Radio Telescope in the Netherlands. In the observing mode of the Large European Array for Pulsars, the telescopes of the European Pulsar Timing Array are tied together to synthesize a fully steerable 200-m dish to greatly enhance the sensitivity of the array towards gravitational waves.

As, Dr. Antoniadis explained, radiation beams from the pulsars’ magnetic poles circle around their rotational axes, and they are observed as pulses when they pass our line of sight, like the light of a distant lighthouse. Pulsar timing arrays are networks of very stably rotating pulsars, used as galactic-scale gravitational wave detectors. In particular, they are sensitive to very low frequency gravitational waves in the billionth-of-a-Hertz regime. This will extend the gravitational wave observing window from the high frequencies (hundreds of Hertz) currently observed by the ground-based detectors LIGO/Virgo/KAGRA. While those detectors probe short lasting collisions of stellar-mass black holes and neutron stars, Pulsar Timing Arrays can probe gravitational waves such as those emitted by systems of slowly in-spiraling supermassive black-hole binaries hosted at the centres of galaxies. The addition of the gravitational waves released from a cosmic population of these binaries forms a gravitational wave background.

The EPTA can measure small fluctuations in the arrival times of the pulsars’ radio signal at Earth, caused by the spacetime deformation due to a passing-by very low frequency gravitational wave. In practice, these deformations manifest as sources of a very low frequency noise in the series of the observed times of arrival of the pulses, a noise which is shared by all the pulsars of a Pulsar Timing Array.

However, the amplitude of this noise is incredibly tiny (estimated to be tens to a couple of hundreds of a billionth of a second) and in principle many other effects could impart that to any given pulsar in the Pulsar Timing Array.

To validate the results, multiple independent codes with different statistical frameworks were then used to mitigate alternate sources of noise and search for the gravitational wave background. Importantly, two independent end-to-end procedures were used in the analysis for cross-consistency. Additionally, three independent methods were used to account for possible systematics in the Solar-system planetary parameters used in the models predicting the pulse arrival times, a prime candidate for false-positive gravitational wave signals.

The European Pulsar Timing Array analysis with both procedures found a clear candidate signal for a gravitational wave background and its spectral properties (i.e. how the amplitude of the observed noise varies with its frequency) remain within theoretical expectations for the noise attributable to a gravitational wave background. The European Pulsar Timing Array first found indications for this signal in their previously published data set in 2015, but as the results had larger statistical uncertainties, they were only strictly discussed as upper limits. The new data now clearly confirm the presence of this signal, making it a candidate for a gravitational wave background.

Einstein’s General Relativity predicts a very specific relation among the spacetime deformations experienced by the radio signals coming from pulsars located in different directions in the sky. Scientists call that the spatial correlation of the signal, or Hellings and Downs curve. Its detection – which was not possible in this study – will uniquely identify the observed noise as due to a gravitational wave background.

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” 2021; The EPTA collaboration (incl. J. Antoniadis), MNRAS, 508, 4 , 4970 - October 2021