Nano-Hz gravitational wave signature from axion dark matter
  We calculate the accurate spectrum of the stochastic gravitational wave
background from U(1) gauge fields produced by axion dark matter. The explosive
production of gauge fields soon invalidates the applicability of the linear
analysis and one needs nonlinear schemes. We make use of numerical lattice
simulations to properly follow the nonlinear dynamics such as backreaction and
rescattering which gives important contributions to the emission of
gravitational waves. It turns out that the axion with the decay constant $f
\sim 10^{16}$ GeV which gives the correct dark matter abundance predicts the
circularly polarized gravitational wave signature detectable by SKA. We also
show that the resulting gravitational wave spectrum has a potential to explain
NANOGrav 12.5 year data.
