« Pulsar Timing arrays: first evidence of a gravitational wave signal at low frequency »
Since the 2000s, most of the world’s major radio telescopes have accumulated arrival time measurements for several tens of millisecond pulsars, with timing accuracy reaching a few tens of nanoseconds for some of the targets. This precision, combined with a high cadence and a long span of observations, makes pulsar networks sensitive to a gravitational wave signal amplitude of a few 10-16 in the low frequency domain between nanoHz and microHz. This is the amplitude expected, for example, from the emission produced by pairs of supermassive black holes host in the core of large galaxies or by various sources in the primordial universe, such as cosmic string loops, relics of inflation or turbulence at the QCD epoch. In June 2023, the first evidences of a gravitational wave signal in this low frequency domain were carried out jointly by four independent consortia in Europe, the United States, Australia and China, each using a different set of radio telescopes and a different pulsar array. These results have been recently confirmed using the first five years of observation with the MeerKAT radio telescope and precursor of SKA. A full (>5-sigma) detection should be achieved in the coming months by combining all the existing data sets under the umbrella of the International Pulsar Timing Array (IPTA), which will allow us to obtain greater sensitivity and better sky coverage. In this seminar, I will discuss the methodology of pulsar timing and detail the complexity of the analysis up to the characterization of the gravitational wave signal. In particular, I will show how this signal, if attributed to the population of super massive black hole binaries, can provide a better understanding of the characteristic rates of galaxy and supermassive black hole mergers, as well as new constraints on the relative masses of black holes and their host galaxies.