  We explore the dynamics of boson stars in the presence of axionic couplings
through nonlinear evolutions of Einstein's field equations. We show that, for
large axionic couplings, isolated boson stars become unstable, and decay via a
large burst of electromagnetic radiation, becoming less massive and more
dilute. Our full nonlinear results are in good agreement with flat-space
estimates for the critical couplings. We then consider head-on collisions of
sub-critical boson stars and study the electromagnetic and gravitational
signal. Boson stars cluster around the critical point via interactions, and we
argue that mergers will generically be also sources of electromagnetic
radiation, in addition to gravitational waves, which can be used to place
constraints on the axionic coupling if such multimessenger signals are
detected.
