Confronting sound speed resonance with pulsar timing arrays

Abstract

The stochastic signal detected by pulsar timing arrays (PTAs) has raised great interest in understanding its physical origin. Assuming the signal is a cosmological gravitational-wave background produced by overly large primordial curvature perturbations, we investigate the sound speed resonance effect with an oscillatory behavior using the combined PTA data from NANOGrav 15-yr data set, PPTA DR3, and EPTA DR2. We find that the stochastic signal can be explained by the induced gravitational waves sourced by the sound speed resonance mechanism, with the oscillation frequency $f_* \in [1.51, 4.90] \times 10^{-7}$Hz and the start time of oscillation $|\tau_0| \in [2.05, 106] \times 10^7$s