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Published online by Cambridge University Press: 03 April 2025
We give sharp point-wise bounds in the weight-aspect on fourth moments of modular forms on arithmetic hyperbolic surfaces associated to Eichler orders. Thereby, we strengthen a result of Xia and extend it to co-compact lattices. We realize this fourth moment by constructing a holomorphic theta kernel on $\mathbf {G} \times \mathbf {G} \times \mathbf {SL}_{2}$, for
$\mathbf {G}$ an indefinite inner form of
$\mathbf {SL}_2$ over
$\mathbb {Q}$, based on the Bergman kernel, and considering its
$L^2$-norm in the Weil variable. The constructed theta kernel further gives rise to new elementary theta series for integral quadratic forms of signature
$(2,2)$.