# Local conductor bounds for modular abelian varieties

## TL;DR

This paper improves bounds on local conductor exponents for modular abelian varieties with maximal real multiplication, showing these bounds are often sharp and linking divisibility conditions of N to the endomorphism algebra.

## Contribution

It provides sharper bounds for local conductors in the case of maximal real multiplication and characterizes the endomorphism algebra based on divisibility conditions of N.

## Key findings

- Bounds are sharp in many cases.
- The rationality field contains specific cyclotomic fields.
- Divisibility of N influences the endomorphism algebra.

## Abstract

Brumer and Kramer gave bounds on local conductor exponents for an abelian variety $A/\mathbb Q$ in terms of the dimension of $A$ and the localization prime $p$. Here we give improved bounds in the case that $A$ has maximal real multiplication, i.e., $A$ is isogenous to a factor of the Jacobian of a modular curve $X_0(N)$. In many cases, these bounds are sharp. The proof relies on showing that the rationality field of a newform for $\Gamma_0(N)$, and thus the endomorphism algebra of $A$, contains $\mathbb Q(\zeta_{p^r})^+$ when $p$ divides $N$ to a sufficiently high power. We also deduce that certain divisibility conditions on $N$ determine the endomorphism algebra when $A$ is simple.