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We consider cosmic string cusp annihilations as a possible source ofenhancement to the global background radiation temperature in $21$-cm photonsat reionization. A soft photon spectrum is induced via the Bremsstrahlung andSynchrotron emission of electrons borne out of QCD jets formed off the cusp.The maximal energy density background comes from synchrotron induced photonswith a string tension of $G\mu \sim 10^{-18}$. In this instance, the radiationbackground at reionization is heated up by $5\cdot 10^{-3} \,\, K$. We findthat the depth of the absorption trough ($\delta T_b$) in $21$-cm atreionization is altered by one part in $10^4$ from the strings, requiring highprecision measurements to be detectable. This mechanism cannot explain the$\delta T_b$ observed by the EDGES experiment.

On the Possible Enhancement of the Global $21$-cm Signal at Reionization from the Decay of Cosmic String Cusps