Gravitational waves from type II axion-like curvaton model and its implication for NANOGrav result

Abstract

The recent report of NANOGrav is gathering attention since its signal can be explained by a stochastic background of gravitational waves (GWs) with an abundance of ΩGW ∼ 10-9 at the reference frequency f ∼ 10-8.The PBH formation scenario is one of the candidates for the NANOGrav signal, which can simultaneously explain the observed 30M☉ black holes in the binary merger events in LIGO-Virgo collaboration.We focus on the type-II axion-like curvaton model of the PBH formation.In the type II model, the complex field whose phase part is the axion rolls down from the origin of the potential.It is found that type II model achieves the broad power spectrum of the density perturbations and can simultaneously explain the LIGO-Virgo events and the NANOGrav signal.We also improve the treatment of the non-Gaussianity of perturbations in our model to accurately estimate the amplitude of the induced GWs.