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Axion strings are horizon-size topological defects that may be produced inthe early Universe. Ultra-light axion-like particles may form strings thatpersist to temperatures below that of big bang nucleosynthesis. Such stringshave been considered previously as sources of gravitational waves and cosmicmicrowave background (CMB) polarization rotation. In this work we show, throughanalytic arguments and dedicated adaptive mesh refinement cosmologicalsimulations, that axion strings deposit a sub-dominant fraction of their energyinto high-energy Standard Model (SM) final states, for example, by the directproduction of heavy radial modes that subsequently decay to SM particles. Thishigh-energy SM radiation is absorbed by the primordial plasma, leading to novelsignatures in precision big bang nucleosynthesis, the CMB power spectrum, andgamma-ray surveys. In particular, we show that CMB power spectrum dataconstrains axion strings with decay constants $f_a \lesssim 10^{12}$ GeV, up tomodel dependence on the ultraviolet completion, for axion masses $m_a \lesssim10^{-29}$ eV; future CMB surveys could find striking evidence of axion stringswith lower decay constants.

Signatures of Primordial Energy Injection from Axion Strings