Binary black hole coalescence and the dynamics of scalar hair in Einstein-Maxwell-scalar theory

We investigate the head-on coalescence of charged binary black holes in Einstein-Maxwell-Scalar (EMS) theory using numerical relativity. The binaries are built from charged puncture initial data representing two Reissner-Nordström black holes immersed in a purely kinetic scalar perturbation: the scalar field initially vanishes, while its conjugate momentum provides a small seed for the instability. We evolve the coupled gravitational, electromagnetic, and scalar sectors and monitor the apparent horizons, the emitted radiation, and the scalar field on the horizons. Our simulations show that the nonminimal electromagnetic-scalar coupling can dynamically trigger the growth of scalar hair even when the individual black holes are initially scalar-free. The subsequent evolution depends on the coupling parameter $α_0$ and on the charge-to-mass ratio of the remnant. For weak coupling, or when charge cancellation suppresses the electromagnetic source after the merger, the scalar field is radiated away or absorbed by the final horizon, and the system dynamically descalarizes. For sufficiently strong coupling and nonzero remnant charge, the scalar field remains finite, and the final black hole approaches a scalarized configuration. For binaries with equal charge-to-mass ratios, $q_1=q_2=0.1$, our simulations identify a transition region between $1900<α_0<2000$, that separates descalarized and scalarized remnants. The coalescence also excites scalar radiation whose time profile is qualitatively correlated with the dominant gravitational-wave mode during the nonlinear stage of the collision. These results provide a binary realization of scalarization/descalarization transitions in EMS theory and show that the fate of scalar hair is controlled by the interplay between the scalar coupling and the charge content of the remnant.