Parametrization of virtually $K$-rational Drinfeld modules of rank two

TL;DR

This paper introduces virtually $K$-rational Drinfeld modules of rank two over function fields and proves they are parametrized by rational points on certain modular curves, extending concepts analogous to $Q$-curves.

Contribution

It establishes a parametrization of virtually $K$-rational Drinfeld modules of rank two without complex multiplication via $K$-rational points on quotient modular curves.

Findings

All such Drinfeld modules are parametrized by rational points.

The parametrization is up to isogeny.

Analogue of Elkies' result for $Q$-curves.

Abstract

For an extension $K/\mathbb{F}_q(T)$ of the rational function field over a finite field, we introduce the notion of virtually $K$-rational Drinfeld modules as a function field analogue of $\mathbb{Q}$-curves. Our goal in this article is to prove that all virtually $K$-rational Drinfeld modules of rank two with no complex multiplication are parametrized up to isogeny by $K$-rational points of a quotient curve of the Drinfeld modular curve $Y_0(\mathfrak{n})$ with some square-free level $\mathfrak{n}$. This is an analogue of Elkies' well-known result on $\mathbb{Q}$-curves.