Bessel $F$-isocrystals for reductive groups

Abstract: We construct the Frobenius structure on a rigid connection $\mathrm{Be}<em>{\check{G}}$ on $\mathbb{G}_m$ for a split reductive group $\check{G}$ introduced by Frenkel-Gross. These data form a $\check{G}$-valued overconvergent $F$-isocrystal $\mathrm{Be}</em>{\check{G}}<sup>{\dagger}$ on $\mathbb{G}<em>{m,\mathbb{F}_p}$, which is the $p$-adic companion of the Kloosterman $\check{G}$-local system $\mathrm{Kl}</em>{\check{G}}$ constructed by Heinloth-Ng^o-Yun. By exploring the structure of the underlying differential equation, we calculate the monodromy group of $\mathrm{Be}<em>{\check{G}}<sup>{\dagger}$ when $\check{G}$ is almost simple (which recovers the calculation of monodromy group of $\mathrm{Kl}</em>{\check{G}}$ due to Katz and Heinloth-Ng^o-Yun), and establish functoriality between different Kloosterman $\check{G}$-local systems as conjectured by Heinloth-Ng^o-Yun. We show that the Frobenius Newton polygons of $\mathrm{Kl}<em>{\check{G}}$ are generically ordinary for every $\check{G}$ and are everywhere ordinary on $|\mathbb{G}</em>{m,\mathbb{F}_p}|$ when $\check{G}$ is classical or $G_2$.