Uniform polynomial bounds on torsion from rational geometric isogeny classes

In 1996, Merel showed there exists a function $B\colon \mathbb{Z}^+\rightarrow \mathbb{Z}^+$ such that for any elliptic curve $E/F$ defined over a number field of degree $d$, one has the torsion group bound $\# E(F)[\textrm{tors}]\leq B(d)$. Based on subsequent work, it is conjectured that one can choose $B$ to be polynomial in the degree $d$. In this paper, we show that such bounds exist for torsion from the family $\mathcal{I}_{\mathbb{Q}}$ of elliptic curves which are geometrically isogenous to at least one rational elliptic curve. More precisely, we show that for each $\epsilon>0$, there exists $c_\epsilon>0$ such that for any elliptic curve $E/F\in \mathcal{I}_{\mathbb{Q}}$, one has \[ E(F)[\textrm{tors}]\leq c_\epsilon\cdot [F:\mathbb{Q}]^{3+\epsilon}. \] This generalizes work of the second author for elliptic curves within a fixed rational geometric isogeny class. For the family of elliptic curves with rational $j$-invariant, we also obtain bounds which improve those of Clark and Pollack. In this case, our bounds on the exponent of $E(F)[\textrm{tors}]$ are optimal if one does not exclude elliptic curves with complex multiplication.