We consider the nonequilibrium dynamics of a system of interacting massless fermions in a ring threaded by a magnetic flux. We focus on the quench where the flux is initially vanishing and is then turned on. We show that the definition of the limit of abrupt quench is problematic due to the presence of gauge invariance that has to be taken into account. We then propose a specific protocol where the dynamics is nontrivial. Employing techniques coming from the algebraic Bethe ansatz, we present an exact formula for the Loschmidt echo valid at all times as a Fredholm determinant at the free fermionic point. From the analysis of the asymptotic behavior of the Fredholm determinant, we show that the distribution of work done at small energies presents an edge singularity whose exponent can be explicitly computed. Using the correspondence between the edge singularity and the decay of the fidelity at finite size we propose a general formula for the exponent valid also in the interacting case.