21-cm Constraints on S-wave Dark Matter Annihilation with Subhalo-enhanced Effects

Observations of the 21-cm signal can probe energy injection into the intergalactic medium arising from dark matter annihilation during cosmic dawn and reionization. For s-wave annihilation, the annihilation rate depends on the squared dark matter density, making the 21-cm signal sensitive to dark matter halo substructure. In this work, we extend the numerical method of DM21cm to address spatially inhomogeneous s-wave annihilation including subhalo effects, using semi-analytical treatment of SASHIMI-C. Our numerical results show that the projected HERA limit for the $\gamma \gamma$ annihilation channel is stronger than the Leo~T bound within the dark matter mass range of $m_{\chi }\le 3\mathrm{k}\mathrm{e}\mathrm{V}$, whereas the projected HERA limit for the $e^{+}{e}^{-}$ annihilation channel is the strongest among the existing bounds for $m_{\chi }\sim 0.1$–$1\mathrm{G}\mathrm{e}\mathrm{V}$.