Modular forms of CM type mod

We say that a normalized modular form without complex multiplication is of CM type modulo by an imaginary quadratic field K if its Fourier coefficients aₚ are 0 modulo a prime ideal dividing 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 by an imaginary quadratic field K, does there exist a congruence modulo 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 Q-curves completely defined over an imaginary quadratic field, and the case of elliptic curves over Q with modular-maximal cyclic group of order 16 as 5-torsion Galois module. In all these situations, at some specific primes, 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.