Contributions of dark matter annihilation within ultracompact minihalos
  to the 21cm background signal
  In the dark age of the universe, any exotic sources, e.g. the dark matter
annihilation, which inject the energy into the intergalactic medium (IGM) will
left some imprint on the 21cm background signal. Recently, one new kind of dark
matter structure named ultracompact dark matter minihalo (UCMHs) was proposed.
Near the inner part UCMHs, the distribution of dark matter particles are
steeper than that of the general dark matter halos, $\rho_{\rm UCMHs}(r) \sim
r^{-2.25}$, and the formation time of UCMHs is earlier, $z_c \sim 1000$.
Therefore, it is excepted that the dark matter annihilation within UCMHs can
effect the 21cm background signal. In this paper, we investigated the
contributions of the dark matter annihilation within UCMHs to the 21cm
background signal.
