Electromagnetic outflows in a class of scalar-tensor theories: binary neutron star coalescence

As we showed in previous work, the dynamics and gravitational emission of binary neutron star systems in certain scalar-tensor theories can differ significantly from that expected from General Relativity in the coalescing stage. In this work we examine whether the characteristics of the electromagnetic counterparts to these binaries -- driven by magnetosphere interactions prior to the merger event -- can provide an independent way to test gravity in the most strongly dynamical stages of binary mergers. We find that the electromagnetic flux emitted by binaries in these scalar-tensor theories can show deviations from the GR prediction in particular cases. These differences are quite subtle, thus requiring delicate measurements to differentiate between GR and the type of scalar-tensor theories considered in this work using electromagnetic observations alone. However, if coupled with a gravitational-wave detection, electromagnetic measurements might provide a way to increase the confidence with which GR will be confirmed (or ruled out) by gravitational observations.