Blazar Constraints on Axions through New Spectral Modulation Searches in 1ES 1959+650 & B2 1811+31

Blazars are unique astrophysical environments whose high-energy $\gamma$-ray spectra are susceptible to modulations in the presence of ultralight axions. We search for these modulations, induced by axion-photon mixing, in Fermi-LAT spectral data of previously unexplored blazar targets, focusing in particular on blazars 1ES 1959+650 and B2 1811+31, whose flare states provide a clean testbed for axion activity. In both cases, we find no evidence for axions, and set exclusion regions on the axion-photon coupling for masses between ${10}^{-9}$ eV $\lesssim$ $m_a$ $\lesssim$ ${10}^{-8}$ eV, with sensitivities typically reaching $g_{a\gamma \gamma }\sim {10}^{-11}-{10}^{-10}$ GeV${}^{-1}$ depending on the assumed blazar modeling choices. We examine the broad impact of modeling uncertainties, finding that the resulting constraints can vary substantially across plausible configurations. We discuss the implications of these systematic effects and their relevance for similar blazar-like searches in the future.