Constraints on Superconducting Cosmic Strings from the Global $21$-cm
  Signal before Reionization
  Electromagnetic radiation from the cusp region of superconducting cosmic
strings leads to a radio excess in the photon spectrum in the early universe
and can produce a deep absorption feature in the global 21cm signal before the
epoch of reionization. We study the constraints on the parameter space of
superconducting strings which can be derived by demanding that the absorption
feature is not larger in amplitude than what has recently been reported by the
EDGES collaboration.
