Revisiting constraints on superconducting cosmic strings in light of the dark ages global 21-cm signal

The superconducting cosmic strings (SCS) are a special case of cosmic strings that have a core carrying a charged field. When SCS passes through magnetized regions, the charged particles in the string experience a Lorentz force, which can produce radiation on the entire electromagnetic spectrum. This radiation can inject energy into the surrounding plasma, resulting in a modification of the thermal and ionization evolution of the intergalactic medium (IGM) and, subsequently, the global 21-cm signal. The signatures of SCS in the postrecombination era have been primarily studied in the low-frequency (radio) regime, which does not impact the state of the IGM. In this work, we study the effect of decaying SCS on the dark ages global 21-cm signal $(\ensuremath{\delta}{T}_{b})$, considering both the ionizing and radio radiation. The dark ages signal can provide pristine cosmological information free from astrophysical uncertainties, as the Universe was primarily homogeneous during this era in the absence of baryonic structure formation. Considering a change in the $\ensuremath{\delta}{T}_{b}$ at redshift $z\ensuremath{\sim}89$ from the Lambda Cold Dark Matter ($\mathrm{\ensuremath{\Lambda}}$CDM) framework, we derive an upper bound on the decay efficiency parameter, $g\ensuremath{\equiv}g(I,G{\ensuremath{\mu}}_{s})$, to be $\ensuremath{\lesssim}5.1\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }{\mathrm{GeV}}^{2}$, where, $I$ and $G{\ensuremath{\mu}}_{s}$ represent the loop current and string tension of SCS, respectively.