Constraints of relic gravitational waves by pulsar timing arrays: Forecasts for the FAST and SKA projects

Measurement of the pulsar timing residuals provides a direct way to detect relic gravitational waves at the frequency $f\sim 1/{\rm yr}$. In this paper, we investigate the constraints on the inflationary parameters, the tensor-to-scalar ratio $r$ and the tensor spectral index $n_t$, by the current and future Pulsar Timing Arrays (PTAs). We find that Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China and the planned Square Kilometer Array (SKA) projects have the fairly strong abilities to test the phantom-like inflationary models. If $r=0.1$, FAST could give the constraint on the spectral index $n_t<0.56$, and SKA gives $n_t<0.32$. While an observation with the total time T=20 yr, the pulsar noise level $\sigma_w=30$ns and the monitored pulsar number $n=200$, could even constrain $n_t<0.07$. These are much tighter than those inferred from the current results of Parkers Pulsar Timing Array (PPTA), European Pulsar Timing Array (EPTA) and North American Nanohertz Observatory for Gravitational waves (NANOGrav). Especially, by studying the effects of various observational factors on the sensitivities of PTAs, we found that compared with $\sigma_w$ and $n$, the total observation time $T$ has the most significant effect.