A New Picture of Cosmic String Evolution and Anisotropic Stochastic
  Gravitational-Wave Background
  We investigate the anisotropies of the stochastic gravitational-wave
background (SGWB) produced by cosmic strings associated with the spontaneous
U(1) symmetry breaking of Grand Unified Theory, which happens at the onset of
inflation. The string network evolution is determined by primordial
fluctuations and never reaches the scaling regime. The string loops are
inhomogeneously distributed in large scale regions, resulting in large
anisotropies in the SGWB. We find that the angular power spectrum of SGWB
anisotropies depends on frequency, which is testable in multiband observations
of GWs. In particular, GWs from the cosmic strings can appropriately interpret
the common-spectrum process reported by NANOGrav collaboration, and the angular
power spectrum in the nanohertz band,
$\mathtt{l}(\mathtt{l}+1)C_{\mathtt{l}}=5.6\times 10^{-2}$ at large scales, is
expected to be detectable by pulsar timing array experiments in the near
future.
