Searching for ultra-long period gravitational waves with the Pulsar Timing Arrays (PTAs).

A Pulsar Timing Array (PTA) exploits the remarkable rotational stability of an ensemble of rapidly spinning pulsars in order to use them as pieces of a galactic scale gravitational wave detector. In particular, a PTA provide the possibility to search for gravitational waves in the ultra-long period range, between few months to few decades, thus nicely complement other current and future gravitational waves detectors, which are sensitive at much shorter gravitational wave periods. After about 15 years of constant progresses, recent results from various regional PTA efforts (the European EPTA, the North-American nanoGrav, the Australian PPTA and the Indian InPTA), as well as from the combined International Pulsar Timing Array (IPTA), might indicate that a detection is not far in time anymore. A report will be given about the status of the various experiments, with particular focus on the Italian/European contribution.