Cosmic strings produce charged particles which, by emitting electromagnetic radiation, partially ionize neutral hydrogen during the dark ages. Corrections to the ionization fraction of neutral hydrogen induced by cosmic strings lead to new constraints on the string tension around $G\mu \sim {10}^{-16}–{10}^{-20}$ for values of the primordial magnetic field greater than $B_0\sim {10}^{-11}\mathrm{G}$.

• Received 8 August 2019

DOI:https://doi.org/10.1103/PhysRevD.101.023528