Nonminimally coupled scalar field in Schwarzschild--de Sitter spacetime: Geodesic synchrotron radiation

We analyze the scalar radiation emitted by a source interacting with a nonminimally coupled scalar field in four-dimensional Schwarzschild--de Sitter spacetime. We obtain the emission probability using quantum field theory in curved spacetimes at tree level. We find that the source emits synchrotron-type radiation for orbits near the photon sphere for all allowed values of the parameters. We also find that the emitted power strongly depends on the coupling to the curvature scalar. In particular, the previously observed enhancement in the contribution of lower multipoles to the emitted power in this spacetime with minimal coupling is absent when conformal coupling is considered.