Axion Universal Gravitational Wave Interpretation of Pulsar Timing Array Data

Formation of cosmological solitons is generically accompanied by production of gravitational waves (GWs), with a universal GW background expected at frequency scales below that of non-linear dynamics. Beginning with a general phenomenological description of GWs associated with soliton formation, we demonstrate that universal GW background from axion-like particle (ALP) solitonic oscillons provides a viable interpretation to the recent NANOGrav 15 year pulsar timing array data, which does not suffer from the overproduction of primordial black holes. We show that pulsar timing array data displays preference for models where formed solitons do not strongly interact or cluster. Coincidence observations with Nancy Roman telescope will allow to discriminate between distinct scenarios of cosmological solitons.