Modular forms of CM type mod $\ell$

Abstract: We say that a normalized modular form without complex multiplication is of CM type modulo $\ell$ by an imaginary quadratic field $K$ if its Fourier coefficients $a_p$ are $0$ modulo a prime ideal dividing $\ell$ for every prime $p$ inert in $K$. In this paper, we address the following problem: Given a weight 2 cuspidal Hecke eigenform $f$ without CM which is of CM type modulo $\ell$ by an imaginary quadratic field $K$, does there exist a congruence modulo $\ell$ between $f$ and a genuine CM modular form of weight 2? We conjecture that the answer is yes, and prove this conjecture in most cases. We study three situations: the case of modular forms attached to abelian surfaces with quaternionic multiplication, the case of $\mathbb{Q}$-curves completely defined over an imaginary quadratic field, and the case of elliptic curves over $\mathbb{Q}$ with modular-maximal cyclic group of order $16$ as $5$-torsion Galois module. In all these situations, at some specific primes $\ell$, it can be shown that the residual representation is monomial by a quadratic imaginary field $K$ (or even more than one), and we can conclude that in most of these cases there is a congruence with a CM modular form. Finally, we present some of the numerical evidence that initially led us to formulate the conjecture.