  Swarlamators are particles capable of synchronize and swarm. Here we study
the effects produced by an external periodic stimulus over a system of
swarmalators that move in two dimensions. When the particles are fixed and
interact with equal strength (Kuramoto oscillators) their phases tend to
synchronize and lock to the external stimulus if its intensity is sufficiently
large. Here we show that in a system of swarmalators the force also shifts the
phases and angular velocities leading to synchronization with the external
frequency. However, the correlation between phase and spatial location
decreases with the intensity of the force, going to zero in what appears to be
a second order phase transition. In the regime of zero correlation the
particles form a static symmetric circular distribution, following a simple
model of aggregation. Interestingly, for intermediate values of the force
intensity, a different pattern emerges, with the particles splitting in two
clusters centered at opposite sides of the stimulus' location, breaking the
radial symmetry. The two-cluster pattern is stable and active, with the
clusters slowly rotating around the source while exchanging particles.
