On the rationality of algebraic monodromy groups of compatible systems

Abstract: Let $E$ be a number field and $X$ a smooth geometrically connected variety defined over a characteristic $p$ finite field. Given an $n$-dimensional pure $E$-compatible system of semisimple $\lambda$-adic representations of the \'etale fundamental group of $X$ with connected algebraic monodromy groups $G_\lambda$, we construct a common $E$-form $G$ of all the groups $G_\lambda$ and in the absolutely irreducible case, a common $E$-form $G\hookrightarrow\text{GL}{n,E}$ of all the tautological representations $G\lambda\hookrightarrow\text{GL}{n,E\lambda}$ (Theorem 1.1). Analogous rationality results in characteristic $p$ assuming the existence of crystalline companions in $\text{F-Isoc}^{\dagger}(X)\otimes E_{v}$ for all $v|p$ (Theorem 1.5) and in characteristic zero assuming ordinariness (Theorem 1.6) are also obtained. Applications include a construction of $G$-compatible system from some $\text{GL}_n$-compatible system and some results predicted by the Mumford-Tate conjecture.